I blogged yesterday with photos of a cake my friend Erin Lippitz  made for her niece's first birthday party this weekend.
To begin, Erin printed out a bunch of images associated with the book, including photos of the Cat in the Hat, Thing 1 and Thing 2 and the fish from in the book.
She baked three 8-inch round cakes for the base, and three six-inch cakes to make the hat. She popped them out of the pans and refrigerated them overnight so they'd be firm enough to handle.
She shaved the top of each layer with a sharp bread knife to make it completely flat and easy to stack. She then cut each of the smaller cakes in half horizontally, for a total of six thin layers. She cut the layer intended for the top at a 45-degree angle.
She stacked each of these layers - both for the hat and the base - with buttercream. For the hat section, she also offset them a bit, to make a curve to mimic the shape of the hat.
She took that top cut piece (the one she cut at an angle) and rotated it 180 degrees before stacking it. That made the top of the hat slope – that's also how you make a topsy-turvy cake, Erin says. (See photo to the left for how it looked after being stacked.)
Erin then covered both separate stacks with plastic wrap and put them in the fridge overnight. The next day, she took them out and trimmed them around the sides with her bread knife to make them completely smooth. For the top layer, she followed the curve of the offset stacks to make it look even more like a hat. She put a sucker stick in the middle to keep the layers from shifting.
She frosted the base cake completely with white buttercream and while the frosting was still a little wet, she rolled out her red fondant with a silicone roller. She bought red fondant on globalsugarart.com.
To make the stripes for the base, she used a plastic ruler (Erin uses it exclusively for baking) and a pizza cutter to cut the fondant. She used the width of the ruler as her template. She applied each strip to the cake before the fondant dried, and gently pressed it into the cake. She made the strips long enough to almost touch in the middle. The cake hat covered this seam. She used eight strips to achieve the stripey look you see in the photo below. 
She finished off the bottom edge with red and white balls of fondant she made days in advance. Those don't even need to be refrigerated, and she attached them with a little bit of frosting.
To decorate the top, hat cake, Erin started by making the ruffled hat brim.
She used her hands to shape the edge into a slight ruffle and left it to dry for about an hour. Once it dried, she used a black Americolor Gourmet Writer Food Decorating Pen  to add accents so it looked hand-drawn, just like in the book.
She used a large spatula to move the cake on top of the hat brim, which was sitting on a cake board for easy assembly later. The hat section stayed there until Erin was ready to assemble it at the birthday party. Otherwise, it would have been too tall to transport safely.
Then, Erin cut the piece of round, red fondant for the top. She used the cake pan as a template and laid it on top, gently pressing it into the frosting.
She cut strips of red and white fondant and wrapped them around the cake horizontally. (Erin said she doesn't get the best results when covering an entire cake in fondant.) There was a seam Erin turned to the back, and she said no one noticed it.
After the striped fondant dried, Erin used her food marker to add hand-drawn accents to the top of the hat, as well. She made sure the cake was on the brim before this step. That's because if you handle the cake after you use the marker, the coloring will smudge. She formed her niece's name out of white fondant to add to the top of the cake.
After finishing the cakes, she made the bow tie out of red fondant (she wanted to make sure she had both time and room to do so). She used two equally round balls of fondant and shaped it with her hands.
She sculpted the fish's bowl about a week prior out of gum paste and fondant, and marked on it with food marker after it dried.
She also sculpted the fish over a lunch hour with white fondant she turned orange with red and yellow food coloring. She used a photo from the book as a reference, using an oblong piece of fondant. She stuck a finger in the fondant to make a mouth, and once it dried, added details with the food marker.
The night before the party, Erin stuck a sucker stick in the bottom end of the fish and attached it inside the bowl, which was still somewhat soft. She added blue buttercream to the bowl to look like water right before she was ready to display the fish.
It didn't fit on the cake as Erin originally wanted, so she just displayed it on the table with the cupcakes she made to accompany the cake.
Erin made vanilla cupcakes with red wrappers and frosted them with white buttercream. She added blue cotton candy right before the birthday party attendees were ready to eat them. If it's on longer, the cotton candy will dissolve in the frosting.
She made the Thing 1 and Thing 2 labels on her home computer, using InDesign, and attached them to the wrappers with a piece of tape.
For final assembly, Erin picked up the hat cake with a spatula while her husband spotted it. She gently pulled the spatula out and let it land on the bottom, striped layer.
Aren't the results amazing? Let me know if you have any questions, and I will pass them on to Erin.
All photos taken by Erin Lippitz and used with her permission.