To make the crumb: break apart the sponge cakes into small pieces into a mixing bowl. using your hands, break the pieces apart until it becomes a fine crumb. add milk and mix with your hands until it becomes clumpy, resembling pieces of cake batter. Add flour and mix to evenly distribute and lock in that shape. Add sprinkles and mix to combine.

To prepare the cake batter: preheat oven to 175C/350F. Sift flour, baking powder, baking soda and salt into a mixing bowl and toss to combine. Whip sugar with butter using an electric hand mixer or stand mixer for a few minutes until light and fluffy. Add eggs and vanilla and whip again until incorporated. Add sour cream and milk and whip just to mix in. Slowly add the dry ingredients and mix until there are no dry clumps left. Gently fold through the sprinkles.

Grease a rectangular baking dish, I used a 10x7 inch pan, with butter all over and pour in half the batter, spreading evenly over the base. Sprinkle over 1 heaping handful of the crumbs evenly over this layer of batter. Add the other half of batter and spread evenly over the top (doing your best). Add the rest of the crumbs over the top, spreading evenly and gently pressing down.

Bake the cake: bake for 55 mins to 1 hour, or until a knife comes out clean from the center. Let cool completely before removing from the pan.

Dress the cake: once cooled, flip the cake onto a cooling rack to remove it from the pan. In order to do this, place a cooling rack up against the top of the cake and flip to remove it from the pan. In order to get it right-side up, place a cutting board against the back of the cake, then flip it onto the cutting board to get it back right side up. Drizzle with icing and let the icing set before slicing, about 10 mins. Enjoy!