01 - Preheat the oven to 350°F. Grease and line two 8-inch round cake pans with parchment paper.
02 - Whisk together the gluten-free flour blend, baking powder, baking soda, and salt in a medium bowl. Set aside.
03 - Beat the sugar, butter, and oil together in a large bowl until light and fluffy, approximately 2–3 minutes.
04 - Add eggs one at a time, beating well after each addition. Mix in the vanilla extract.
05 - Alternately add the flour mixture and milk to the wet ingredients, beginning and ending with flour. Mix until just combined—avoid overmixing.
06 - Gently fold in ½ cup rainbow sprinkles with a spatula until evenly distributed.
07 - Divide the batter evenly between the prepared pans. Smooth the tops with an offset spatula.
08 - Bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean.
09 - Let cakes cool in pans for 10 minutes, then turn out onto a wire rack to cool completely before frosting.
10 - Beat the softened butter until creamy. Gradually add powdered sugar, beating until smooth. Mix in vanilla, salt, and 2 tablespoons milk. Adjust consistency with additional milk if needed.
11 - Place one cake layer on a serving plate. Spread a generous layer of frosting on top. Position the second layer and frost the top and sides of the cake.
12 - Decorate with extra sprinkles as desired. Slice and serve immediately or store according to instructions.