Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square pan and an 8-inch round pan. Line the bottoms with parchment or wax paper. Grease the paper and dust lightly with flour.

Make the cake: Beat white sugar, butter, vanilla, and almond extract in a large bowl with an electric mixer on high speed until light and fluffy. Beat in eggs, one at a time, until well combined.

Stir flour, cornstarch, baking powder, and salt together in another bowl.

Beat 1 cup dry ingredients into wet ingredients until just combined. Add 1/2 cup milk; beat until combined. Continue adding 1 cup dry ingredients and 1/2 cup milk, beating after each addition, until all are used and batter is smooth. Divide evenly between the prepared pans.

Bake in the preheated oven until a toothpick inserted in the center of each cake comes out clean, 45 to 50 minutes. Cool in the pans for 10 minutes, then remove from the pans and cool completely on wire racks.

Meanwhile, make the frosting: Beat butter and vanilla in a large bowl with an electric mixer on medium speed until combined, about 30 seconds. Gradually add powdered sugar, beating after each addition and adding milk as needed, until frosting is fluffy, smooth, and spreadable.

Remove 1/3 cup frosting and divide between two small bowls. Tint one small portion a dark pink with red food coloring. Tint the other small portion an orange-pink with red and yellow food coloring. Tint the remaining frosting a light pink with red food coloring.

Place the square cake onto a work surface and rotate 45 degrees so it resembles a diamond. Spread a little light pink frosting on the left and right top sides of the diamond. Cut the round cake in half crosswise to make 2 half-moons. Attach the half-moons to the top left and top right sides of the diamond, gently pressing the cake pieces together to adhere. Spread remaining light pink frosting over the assembled cake. Drop spoonfuls of dark pink and pink-orange frosting onto the cake, then drag an offset spatula through the frosting to create a marbled look.

Pat sprinkles onto the sides of the cake. Cover and chill for up to 3 days. Let stand at room temperature for 30 minutes before slicing into 16 pieces.