Preheat oven to 175°C (350°F). Grease three 8-inch round cake pans, line each with a parchment circle, and grease the parchment.

Sift together flour, cocoa powder, baking soda, baking powder, and salt into a large bowl. Whisk to distribute.

In a separate bowl or jug, whisk together eggs, buttermilk, cooled coffee, vegetable oil, and vanilla until blended.

Pour wet ingredients into the dry ingredients and stir gently until smooth and fully combined. Batter will be thin. Do not overmix.

Divide batter evenly between the three prepared pans. Tap each pan twice on the counter. Bake for 30 to 35 minutes until a toothpick inserted in the center comes out with a few dry crumbs.

Cool in pans for 15 minutes, then turn out onto wire racks. Peel off parchment. Cool completely for 60 minutes before frosting.

Beat softened butter on medium-high for 3 to 4 minutes until pale and fluffy. Add sifted powdered sugar in three additions on low speed. Add vanilla, salt, and 3 tablespoons of cream. Beat on medium-high for 2 minutes. Fold in 1 cup of finely crushed Oreo crumbs.

Place one cake layer on a plate or board. Spread a generous layer of Oreo buttercream. Add second layer, repeat. Add third layer. Apply a thin crumb coat over the entire cake and refrigerate 15 minutes. Apply the final layer of frosting, smooth the sides and top. Press Oreo crumbs along the bottom third of the sides. Arrange whole Oreos around the top edge. Scatter extra crumbs across the top.