Rainbow Sheet Cake

Step 1
Gather all ingredients.

Step 2
Preheat the oven to 350 degrees F (175 degrees C). Grease a 9×13-inch baking pan with baking spray and line with parchment paper; spray parchment with baking spray.

Step 3
Prepare Cake: Beat butter in a stand mixer fitted with a paddle attachment on medium speed until creamy, about 30 seconds. Gradually add white sugar; beat until light and fluffy, 2 to 3 minutes.

Step 4
Add eggs, 1 at a time, and beat just until incorporated after each addition, about 1 minute total. Add vanilla and beat until combined, about 15 seconds. Scrape down sides and bottom of bowl using a flexible spatula.

Step 5
Whisk together flour, baking powder, and salt in a medium bowl until combined.

Step 6
Add flour mixture to butter mixture alternately with milk, beginning and ending with flour mixture, beating on low speed just until combined after each addition.

Step 7
Divide batter equally between 6 small bowls. Add one to two drops of food coloring to each bowl, tinting one bowl red or pink, one orange, one yellow, one green, one blue and one purple. Mix gently to combine and add additional coloring as needed to reach desired shades.

Step 8
Dollop the batter into the prepared pan, alternating colors, until the whole pan is filled. [If you have leftover batter, top up existing dollops of the same color ? don't make a second layer of different colors]. Gently bang the pan on the counter to ensure that the batter is evenly distributed and that all gaps are filled.

Step 9
Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 30 minutes. Cool in pan for 15 minutes. Remove cake from pan and cool on a wire rack until completely cool, at least 1 hour.

Step 10
Prepare frosting: Beat cream cheese and butter in a stand mixer fitted with a paddle attachment on medium speed until smooth, about 1 minute. Reduce mixer speed to low, and gradually add powdered sugar, beating until incorporated, about 2 minutes. Add vanilla and salt, then increase mixer speed to medium, and beat until fluffy, about 1 minute.

Step 11
Spoon out 1/4 cup of frosting into each of 3 small bowls. Add one to two drops of food coloring to each bowl, tinting one bowl red or pink, one orange, and one green. Mix well to combine and add additional coloring as needed to reach desired shades. Transfer each of the colored frostings to piping bags fitted with small round writing tips. Leave the remaining frosting in the mixing bowl white.

Step 12
Using a serrated knife, trim the edges of the cake.

Step 13
Frost the top of the cake with the white frosting, leaving the sides uncovered.

Step 14
Use the colored frosting to pipe small rainbows across top of cake.