In a large mixing bowl, beat the softened butter with a hand mixer or stand mixer on medium speed until creamy and pale, about 2–3 minutes.
Add the sifted cocoa powder and salt, and continue beating on low until just combined.
Gradually add the sifted powdered sugar, about 1 cup at a time, beating on low speed after each addition to prevent a sugar cloud.
Mix in the vanilla extract and 2 tablespoons of cream or milk, then increase speed to medium-high and beat for 1–2 minutes until smooth.
Add more cream or milk, 1 tablespoon at a time, until the frosting reaches your desired consistency—thick enough to pipe, but soft enough to spread.
Beat frosting for another 2–3 minutes until light and fluffy.
Use immediately to frost cakes or cupcakes, or store in an airtight container in the fridge for up to 1 week. Let it come to room temperature and re-whip before using.