In a small saucepan, combine the cocoa powder and butter. Melt together over low heat, stirring constantly until smooth (about 5 minutes).

Remove from heat once the mixture is completely smooth and glossy.

Gradually whisk in the powdered sugar, alternating with small amounts of milk to prevent lumps.

Add the vanilla extract and stir to incorporate.

Test the consistency by lifting your spoon - the glaze should flow in a ribbon and hold its shape for a moment before melting back into the mixture.

If needed, adjust thickness with additional milk (for thinner glaze) or powdered sugar (for thicker glaze).