Preheat oven to 350F. Grease a 9x9 pan and line with parchment paper, if desired. Set aside.
In a large bowl, whisk together the flour, cocoa powder, black cocoa powder, baking powder, baking soda, salt.
Add in the rest of the cake ingredients; sugar, vegetable oil, egg, coffee, milk, and vanilla extract. Whisk until everything is combined and the cake batter is smooth.
Pour the batter into the prepared pan. Pop in the oven and bake for 28-32 minutes. A tooth pick should come out clean when inserted. Allow the cake to cool for about 10 minutes in the pan, then carefully transfer it to a cooling rack to finish cooling completely.
To make the frosting, cream the butter on medium speed for about 2 minutes.
Add in the powdered sugar, salt, and vanilla extract and begin beating everything together. Slowly pour in the ⅓ cup of heavy cream mix until combined and the frosting is fluffy. If you want your frosting a little thinner, add a few more tablespoons of heavy cream.
Lastly, add in the crushed Oreo cookie crumbs. Once added, beat the frosting on a medium speed for about 2 minutes until it has become super light and fluffy.
Top your cooled cake with the frosting and use a knife or offset spatula to spread it evenly. Top with more Oreo crumbs, if desired.