01 - Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, cream the softened butter and granulated sugar using an electric mixer until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, ensuring thorough incorporation after each addition. Mix in vanilla extract and lemon zest, if using.
05 - Alternately add the flour mixture and milk to the creamed butter mixture, starting and ending with the flour. Mix only until just combined to avoid overdevelopment.
06 - Divide the batter evenly between the prepared cake pans.
07 - Bake for 30-35 minutes, or until a toothpick inserted into the center emerges clean.
08 - Allow cakes to cool in their pans for 10 minutes before inverting them onto a wire rack to cool completely.
09 - Beat softened butter until smooth. Gradually add sifted powdered sugar, followed by milk and vanilla extract, beating until the frosting is light and fluffy. Divide frosting into separate bowls and tint each with a different food coloring gel.
10 - Place one cake layer on a serving plate. Spread a layer of colored frosting over the top. Position the second cake layer atop the first, and then frost the entire cake with a base color.
11 - Unleash your creativity! Use the remaining colored frosting to pipe playful designs, shapes, or swirls. Adorn with an assortment of sprinkles, candy eyes, mini marshmallows, gummy candies, wafer sticks, chocolate chips, and cotton candy as desired.