In a large mixing bowl, beat the softened butter on medium speed until smooth and creamy, about 1 minute.

Add the mascarpone cheese and beat just until combined and smooth; avoid overmixing to prevent curdling.

Sift the powdered sugar and cocoa powder together to remove any lumps, then add to the bowl.

Start mixing on low speed until the dry ingredients are incorporated, then increase to medium-high and beat until fluffy, 2–3 minutes.

Beat in the vanilla extract and a pinch of salt. If the buttercream is too thick, add heavy cream or milk 1 tablespoon at a time until you reach a smooth, spreadable consistency.

Use immediately to frost cupcakes, fill layer cakes, or pipe decorations. If not using right away, refrigerate, then bring to room temperature and re-whip before using.