Brush 3 (9-inch) round pans with melted butter and line with parchment paper. Brush with more butter. You could also use non-stick cooking spray. Set aside.

In a medium bowl, measure out flour, baking powder and salt. Whisk together. Set aside.

In a large bowl, cream butter with an electric mixer until smooth. Add sugar and mix. Add eggs and mix until combined. Add half of the flour mixture and half of the milk, plus the vanilla. Mix on medium low until combined. Repeat with remaining flour and milk. Mix until combined, then continue mixing on medium low for 30 seconds. Divide batter between the three prepared pans. Bake on the middle rack of your oven for 22-25 minutes. At 22 minutes, test one of the cakes by inserting a toothpick. If it comes out clean, they are done. If not, continue to bake for just a few minutes more. Let cool completely.

To make the frosting, mix together the butter and shortening until combined. Slowly add 5 cups (575g) of the powdered sugar, mixing between additions until smooth. Add the vanilla extract, cocoa powder and water or milk. Slowly add the remaining 5 cups (575g) and mix until smooth.

To put the cake together, use a large serrated knife to remove the domes from the tops of the cakes so they are flat. Place the first cake on a serving plate or a cardboard cake round. Spread one cup (284g) of frosting in an even layer on top of the cake. Repeat this process with the second layer of cake and frosting, then add the final layer of cake on top.

Smoothly frost the cake. Add sprinkles to the lower half of the side of the cake. Place 2 cups of frosting in a large pastry bag fitted with a 1M pastry tip. Pipe swirls of icing around the top of the cake. Add sprinkles. Store remaining icing in an airtight container in the refrigerator for up to 10 days. Store the cake at room temperature for one day or covered in the refrigerator for up to 3 days.