Preheat oven to 350F. Line the bottom of each cake pan with parchment paper.
Sift the flour, baking powder, and salt into a medium bowl and whisk together. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar and vanilla on medium speed until light and fluffy, about 8 minutes.
Add the eggs, two at a time, beating until each addition is incorporated before adding the next. Scrape down the sides of the bowl with a spatula when necessary.
Add the flour mixture in four parts, alternating with the milk in three parts, beating just until each addition is incorporated before adding the next; do not overmix.
Fold your sprinkles into the cake batter gently - do not overmix or else the sprinkle coloring will spread.
TIP: use about ¼ cup of sprinkles for every 3lbs of batter (one batch of this recipe).
Scrape the batter into the prepared pan or pans and spread it so that it is smooth in the pan(s). Bake until a toothpick inserted in the center comes out clean, about 40 minutes for the 9 x 13-inch pan or minutes for 45 minutes for the 8-inch pans, rotating the pan(s) halfway through.
Transfer to a wire rack and let cool completely in the pan(s). Cover tightly with plastic wrap and refrigerate overnight. Loosen the edges with a straight spatula, invert the pan(s) to remove the cake, and peel off the parchment.