01 - Preheat your 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 unsalted butter and granulated sugar with an electric mixer until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, ensuring each is well beaten before adding the next. Mix in vanilla extract and lemon zest, if using.
05 - Alternately incorporate the flour mixture and milk into the butter mixture, beginning and ending with the flour. Mix only until just combined.
06 - Divide the batter evenly between the two prepared cake pans.
07 - Bake for 30–35 minutes, or until a toothpick inserted into the center emerges clean.
08 - Allow the cakes to cool in their pans for 10 minutes before inverting them onto a wire rack to cool completely.
09 - Beat the softened butter until smooth. Gradually add the sifted powdered sugar, followed by milk and vanilla extract, beating until the frosting is light and fluffy. Divide the frosting into multiple bowls and tint each with a different food coloring gel.
10 - Place one cooled cake layer on a serving plate. Spread a layer of colored frosting over the top. Position the second cake layer atop the first, 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 assorted sprinkles, candy eyes, mini marshmallows, gummy candies, wafer sticks, chocolate chips, and cotton candy as desired.