Preheat oven to 350F and grease and flour three 6” cake rounds.

Whisk together flour, sugar, baking soda, salt, and baking powder in a bowl.

Add butter, and beat well with a hand mixer or in a stand mixer with the whisk attachment, until the mixture resembles fine crumbs.

In a separate bowl, whisk together egg, sour cream, vanilla, and oil until combined.

Pour wet ingredients into the flour and butter mixture, and beat on medium for 3-4 minutes. The mixture will be dry at this point.

Add in water and mix until just combined. Fold in the crushed Oreos. Do not overmix.

Spread batter evenly into prepared pans. Smooth the tops with a spatula.

Bake for approx. 20-25 mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on a wire rack to cool for 20 mins then turn out onto wire rack to cool completely before removing from pan.

Place egg whites and sugar into the bowl of a stand mixer, whisk until combined. 
Place the bowl over a hot water bath on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch or until it reaches a temperature of 165 degrees F on a candy thermometer. 
Place the bowl on your stand mixer and whisk on medium-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to a paddle attachment. Slowly add cubed butter and mix until smooth. Add vanilla and continue to whip until smooth.

Spread some buttercream on the cake board.

Place one layer of cake on a cake board and place it in the fridge for 5-10 minutes. This will help the cake stick to the cake board and not slip as you stack and frost.

Remove from the fridge.

Repeat with remaining layers and apply a thin coat of frosting all over the cake for the crumb coat. Chill for 20mins.
Frost and smooth the sides, leaving a rim at the top of the cake

Place cake in the fridge for 30 minutes.

Once chilled, paint the top edge of the cake with edible gold paint.

Place cut Oreo cookies on the bottom half of the cake, leaving space.

Fill the spaces with sprinkles and gold leaf, as desired.

Enjoy!