1. Preheat oven to 350°F. Place a parchment baking cup in 16 wells of muffin pans, greasing around the inside top of the well to allow for easier removal.
2. Beat cake mix and ¾ cup of butter in a large bowl with an electric hand mixer or in a stand mixer with paddle attachment. Add the oil, milk, coffee, eggs, and 2 teaspoons vanilla and beat with hand mixer or stand mixer on high speed until silky smooth. Fill each baking cup about three-quarters full.
3. Bake for 16-20 minutes, or until a toothpick inserted into the center comes out clean.
4. Once cupcakes are baked, remove from oven and allow them to cool in the pan on wire racks for 10 minutes. Remove cupcakes from pans and allow to cool completely on wire racks, about 15 minutes more.
5. While the cupcakes cool, prepare the buttercream. In a small bowl mix the heavy whipping cream, 1 teaspoon vanilla, and 1 tablespoon of coffee grounds together, then strain through a fine mesh strainer to remove most of the coffee grounds. In a separate mixing bowl, cream 1 cup of butter for 1 minute with a hand held or stand mixer, then add the heavy cream mixture into the butter and mix until incorporated. Add the salt, cocoa powder if using, and 1 cup of powdered sugar at a time while continuing to mix until all ingredients are blended to a smooth consistency.
6. Frost your cupcakes with the buttercream. Store covered in the refrigerator if not serving immediately.