Position rack in the middle of the oven. Preheat oven to 350F. Coat the insides of three 6-inch cake pans with non-stick cooking spray, line the bottoms with parchment paper rounds, then spray parchment.
Whisk together sifted flour, baking powder and salt in a medium size bowl to combine. Set aside.
In a separate bowl beat the butter until creamy, about 2 minutes, with an electric mixer on medium-high speed. Add sugar gradually and beat until very light and fluffy, about 3 minutes, scraping down the bowl once or twice. Beat in the vanilla.
Beat in the eggs one at a time, scraping down the bowl after each addition.
Add the flour alternately with the milk, beating on low speed. Begin and end with flour, and beat briefly until smooth.
Fold in one jar of the sprinkles. Divide the batter evenly between the 3 cake pans and smooth the tops with a spatula.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. The layers will be a light golden brown around the edges and the top, and will have come away from the sides of the pans.
Cool the pans on a wire rack for 8-10 minutes. Remove the cake from the pans, and place the layers directly on the racks to cool completely.