Cup of Yum
1. Oven temperature 350℉ (175℃).
2. Grease, and flour two 8" (20 cm) cake tins. You can line the bottom with parchment paper as well if desired.

Prepare the Pastry Cream (before, or while the cakes are baking)

Cup of Yum
1. Make the pastry cream according to the crema pasticcera recipe below but use only 12 oz (350 ml) of milk. The rest of the ingredients remain the same.
2. Once the pastry cream is made and chilled, fold in the Grand Marnier and whipped cream, then cover and  refrigerate once again.

Make the Cakes

1. With ingredients at room temperature, beat the eggs and sugar in a stand mixer (or with hand beater) until super thick and pale (between 5-10 minutes, depending on the mixer). 
2. Fold flour into mixture a little at a time (by hand), carefully keeping in as much air as possible.
3. Gently spoon into prepared cake pans and place into the pre-heated oven.
4. Bake the cakes for 20-25 minutes. Remove from oven and allow to cool in pan for 15 minutes.
5. Remove from pan, place on a cooling rack, then peel off the parchment paper. Allow to cool completely. It does not matter which side is on the rack as the entire cake is covered with cake pieces.
6. Gently pour the batter into the 2 prepared cake pans.
7. Bake the cakes for 20-25 minutes. Remove from oven and allow to cool in pan for 15 minutes. Remove from pan, place on a cooling rack, then peel off the parchment paper. Allow to cool completely.

Make the Syrup

1. Simmer the sugar and water for about 3 minutes.
2. Add the Grand Marnier and set aside to cool.

Prepare the First Cake to Assemble

1. Trim off the top and bottom browned parts of both cakes. This is similar to taking crust off bread for tea sandwiches.
2. Cut one sponge cake in half with a bread knife (this one will be the main cake so choose the nicest of the two).
3. Brush the top of each of these two layers with the syrup.
4. Place one layer on a cake plate or serving tray and cover with some of the pastry cream (enough for a cake filling). Next, cover the top and sides with the remaining pastry cream.

Prepare the Second Cake to Assemble

1. Slice the other cake horizontally into three layers, then cut each layer into long, thin strips.
2. Cut each strip into tiny cubes which will be used to decorate the cake.

Assemble and Decorate the Mimosa Cake

1. Begin decorating the cake by dropping cubes onto the top of the sponge cake.
2. Continue adding the cubes and pushing them onto the sides and top of the cake so that they cover the cake completely. It doesn't take too long, and isn't difficult to do.
3. Refrigerate the cake for at least 2 hours so that it can set, however, be sure to cover it to keep it from drying out. As much as I hate cling film, this is the best product for the job.
4. Serve the cake as-is, or dust with powdered sugar, which I prefer. It gives it a more "finished" look.