Brownie in a Mug

Bahan-bahan (1 porsi)

• 2 tablespoons unsalted butter
• 3 tablespoons semi-sweet chocolate chips
• 2 tablespoons (packed) dark brown sugar
• 1/8 teaspoon vanilla extract
• Pinch salt
• 1 egg yolk
• 3 tablespoons all-purpose flour
• Vanilla ice cream or whipped cream,