Preheat the oven to 350 degrees F. Spray a 9x13 inch baking pan with nonstick backing spray.
In a bowl, whisk together the flour and baking powder.
Using your electric mixer fitted with the paddle attachment, beat the softened butter and sugar together until fluffy, about 5 minutes. Beat in each egg 1 at a time. Beat in the vanilla extract until combined. Add the flour mixture to the batter about ½ cup at a time, beating on medium-low speed, until combined and smooth. Use a spatula to gently but quickly stir the sprinkles into the batter – don’t overmix! Pour the batter into the greased baking pan.
Bake the cake 30 minutes, or until the edges are slightly golden and the center is set. Let the cake cool completely, then poke holes all over the cake with a fork. Be sure to leave the cake IN the pan so it can catch all the milk!
In a large measuring cup or bowl, whisk together the whole milk, condensed milk and coconut milk. Pour the milk mixture all over the top of the cake, evenly distributing the milks. I like to refrigerate the cake for at least 30 minutes at this point before frosting and serving. You can also make it the night before serving, but I would wait to frost and sprinkle on top until right before serving.