In microwave safe bowl combine bring butter, cocoa, and milk. (Use a bowl bigger than you think you need)
Microwave on 50–70% power in 30–45 second increments, whisking after each.
Continue until butter is fully melted and mixture is smooth and just starting to bubble (usually ~2–3 minutes total).
Whisk in vanilla extract and powdered sugar, 1–2 cups at a time, until glossy and pourable.
Pour over warm cakes right away — it sets quickly as it cools.
○ For a thicker, fudgier glaze → stick closer to 5 ½ cups powdered sugar.
○ For a classic pourable sheet cake glaze → closer to 5 cups (and add a splash of milk if needed).