Place one cake, flat side up, on plate. Spread 1-1/2 cups buttercream over top. Place cake, flat side down, on top. Trim edges. Cut in 4 squares.
Divide remaining buttercream into batches; tint with different colors. Spread a different-colored buttercream over each side of blocks. Using decorating icing, pipe letters on sides; decorate edges.
1. Heat oven to 350°F. Grease two 9 x 2-inch round cake pans, or two 9 x 9 x 2-inch square cake pans, or one 13 x 9 x 2-inch cake pan. Line with waxed paper; grease paper.
2. Stir together flour, baking powder and salt in medium-sized bowl.
3. Beat together butter and sugar in large bowl until pale and fluffy, about 5 minutes, scraping down sides of bowl with rubber spatula as needed. Beat in eggs, one at a time, until blended. Beat in vanilla. Alternately stir in flour mixture and milk, beginning and ending with flour, until just mixed. Pour into preapred pans, dividing equally.
4. Bake in 350°F oven: 9-inch round pans, 25 to 30 minutes; 9-inch square pans, 35-40 minutes; 13 x 9 x 2-inch pan, 45 to 50 minutes; or until toothpick inserted in center comes out clean. Cool cakes in pans on rack for 5 minutes. Remove cakes to rack to cool completely.
Beat confectioner's sugar, butter, heavy cream and vanilla together in large bowl until smooth. Keep covered in plastic wrap until ready to use. Makes about 3 cups frosting.
