Preheat the oven to 350 degrees F.

Place the sugar, flour, cake flour, cocoa, baking powder, baking soda, and salt in a large mixing bowl and stir to combine.

Mix in the butter on low speed, until the mixture resembles moist crumbs (about 2 minutes).

Stir in the eggs,one at a time, scraping the bottom and sides of the bowl with a silicone spatula after each addition.

Add the Greek yogurt and vanilla, scraping the bottom and sides of the bowl to be sure all the ingredients are well-combined.

Beat on medium speed for 1 to 1 1/2 minutes, to aerate the batter and build the cake’s structure.

Fill lined cupcake wells just over half full (approx. 1/4 cup of batter).

Bake for 15 minutes, or until a cake tester inserted in the center of a cupcake comes out clean or with moist crumbs.

Cool completely before frosting.

Place the softened butter and salt in a large mixing bowl and beat together until creamy.

Add the powdered sugar, 1 1/2 cups at a time, mixing on low speed until fully incorporated.

When all the powdered sugar has been mixed in, add the vanilla extract, turn the mixer speed up to medium-high, and whip until fluffy (about 3-5 minutes).

Pour the heavy cream down the side of the mixing bowl, in a slow and steady stream, while whipping.

Add the crushed Oreo cookie crumbs, folding in gently until just barely combined.

Transfer the frosting to a piping bag fitted with a use a 4B tip.

Pipe the frosting in a swirl on each cupcake, and garnish with an Oreo cookie.