Preheat oven to 350 degrees F.
Line an 11-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
Using a mixer, cream together butter and sugar until fluffy, or for about 3 minutes. On low speed, add eggs one at a time, add vanilla extract and beat until combined, making sure to scrape the sides of the bowl with a spatula. Combine flour, baking powder and salt in a bowl. Add half of the dry ingredients to the wet ingredients, mixing until just combined, add the milk and once combined, add remaining dry ingredients.
Using a spatula, fold in assorted sprinkles. Pour into prepared pan and and bake for 50 - 60 minutes, or until a cake tester inserted into the middle of the cake comes out clean. Half time into baking, rotate the pan, to make sure the cake bakes uniformly.
Let the cake cool into the pan for 10-15 minutes, before transferring to onto a wire rack. Let the cake completely cool before topping with white chocolate.
Let the cake cool completely before drizzling with white chocolate.
Bring a medium saucepan half filled with water to a boil. Place white chocolate in a medium bowl set over the saucepan of simmering water, let it melt, stirring just a few times. Make sure the bowl doesn't touch the boiling water, to avoid overheating. Remove bowl from heat and pour the white chocolate over the cake, top with sprinkles.
Let stand for 15-20 minutes before serving for the chocolate to settle, or better place cake in the fridge for 10 minutes for the chocolate to harden quicker.