1 tablespoon unsalted butter
A few drops vanilla extract
3 to 4 tablespoons granulated sugar, to taste
3 tablespoons unsweetened cocoa powder
A pinch of kosher salt
Microwave the butter in a microwave-safe mug in 10-second increments until melted, 20 to 30 seconds. Crack the egg into a small bowl; add the vanilla and whisk to combine using a small whisk or fork. Pour the egg mixture into the mug over the butter and whisk to combine.
Add the sugar, cocoa powder and salt and whisk until mostly smooth (a few lumps are okay). Scrape down the sides the best you can with a spoon or a small silicone spatula. Sprinkle with mini chocolate chips, if desired.
Cook in the microwave on high for 1 to 1 ½ minutes, or until a toothpick inserted in the centre comes out mostly clean. Sift a little icing sugar on top or serve with a scoop of vanilla ice cream, if desired. Dig in with a spoon.