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Room temperature ingredients are non-negotiable for perfect batter. Cold butter and eggs can cause curdling, resulting in a dense cake. To speed up softening, cube the butter and set aside for 15–20 minutes. Cream the butter and sugar until fluffy, then incorporate eggs one at a time, adding vanilla for flavor.
Mixing and Incorporating the Rainbow Sprinkles
Alternately add dry ingredients and milk—being careful not to overmix or undermix. Overmixing develops gluten, leading to a chewy cake, while undermixing leaves dry pockets. Once just combined, gently fold in half a cup of rainbow sprinkles, ensuring not to stir vigorously to prevent color bleeding and gray batter.