Combine the brown sugar, cocoa powder, cornstarch and salt in a medium saucepan until no lumps remain.
1/2 cup (100 grams) brown sugar, 1/4 cup (29 grams) cocoa powder, 1/4 cup (31 grams) cornstarch, 1/4 teaspoon salt
Stir in the milk, turn the heat to medium, and while stirring occasionally, bring to a boil. Let boil for up to about a minute or until the pudding coats the back of a spoon. Do not let it boil too long or it'll break down later. It's important to not taste the pudding with a spoon and put it back in the pudding! It'll make the pudding break down into soup.
2 3/4 cups (650 ml) full-fat milk
Remove the pan from the heat and stir in the butter and vanilla extract. Let cool to room temperature, stirring every 15 minutes or so, then place plastic wrap over the pudding to prevent a skin from forming. Let chill for about 2-3 hours and then serve.
2 tablespoons (28 grams) butter, 1 teaspoon vanilla extract
Store covered in the fridge for up to 4 days.