On the stove or in a microwave-safe bowl, melt the butter. Stir through the cocoa powder until it has all been combined into a glossy and smooth mixture.

Add in the vanilla, salt, powdered sugar, and 2 tablespoons of milk or cream. If your powdered sugar is clumpy, sift it in to avoid lumps in the icing.

Use an electric hand mixer or a balloon whisk or wooden spoon to beat the ingredients together until smooth. Add more milk to make the icing thinner as needed.

Spread over cooled brownies.

Leftover icing can store at room temperature in an airtight for up to 2 days. The butter will stiffen up, so whip it together with an electric mixer to get it spreadable again.