Instructions

1. Funfetti cake
   Grease and line a quarter sheet pan (9 by 13 inches) with baking or parchment paper. Preheat oven to 180 C (350 F) standard / 160 C (320 F) fan-forced.
2. In a large mixing bowl, add butter, sugar and vanilla and beat with an electric mixer on medium speed until pale and creamy. Add eggs, one at a time, and beat after each addition.
3. In a separate bowl, add flour, baking powder and salt, and whisk briefly. Add half of the flour mix and all the milk and beat on a low speed until combined.
4. Add remaining flour mix and sour cream and continue to mix on a low speed until just combined and a thick, creamy cake batter forms. Gently fold in sprinkles by hand, being careful not to overmix. 
5. Pour cake into prepared pan and smooth the top. Bake for approximately 25-30 minutes or until golden and a skewer inserted in the middle comes out clean. Set on a wire rack to cool completely.
6. Cookie crumbs
   Preheat oven to 180 C (350 F) standard / 160 C (320 F) fan-forced. 
7. In a large mixing bowl, add flour, sugar and cornflour. Whisk together. Pour over melted butter, along with the vanilla, stir briefly. Add sprinkles and then stir until it forms wet crumbs that look like wet sand.
8. Pour onto a baking tray lined with baking paper and spread out into one layer. Bake for 10 minutes, then leave to cool completely. 
9. Vanilla frosting
   Place butter in a large mixing bowl. Beat with an electric mixer on medium speed for 1-2 minutes until butter is smooth and creamy. 
10. Add vanilla and 125 grams (1 cup) sugar. Beat on a low speed to combine, then turn up the speed to medium and mix for 30 seconds. Next, add 250 grams (2 cups) sugar, along with the Greek yogurt.
11. Beat on medium speed until combined. Scrape down the sides of the bowl. Add final 125 grams (1 cup) sugar, milk and baking powder. Beat on medium speed until frosting is fluffy and creamy.
12. Assemble cake
    Gently remove cooled cake from pan and remove baking paper. Cut out three 6-inch layers using a 6-inch cake ring. Note the diagram below – you’ll get two whole layers and then two halves.
13. Place a 6-inch cake ring on a cake board or plate. Line the inside of the cake ring with a sheet of acetate. Place the two half pieces of cake at the bottom. Press down the cake firmly. If there are any gaps, use extra cake scraps to fill in the holes and press down very firmly to make one even layer.
14. Spoon 2-3 tablespoons of the milk soak (milk and vanilla combined) over the first cake layer. Add a few tablespoons of vanilla buttercream and spread into one layer. Scatter over a handful of cookie crumbs. Add a few extra tablespoons of frosting on top. You don’t need to completely cover the crumbs.
15. Add another layer of cake and press down firmly. Spoon 2-3 tablespoons of the milk soak over the cake layer. Add a few tablespoons of vanilla buttercream and spread into one layer. Scatter over a handful of cookie crumbs. Add a few extra tablespoons of frosting on top.
16. Add final cake layer and press down firmly. Add remaining buttercream and scatter over cookie crumbs. Place cake in the fridge for at least 1 hour to set.