Chocolate Chip Cookie In A Mug

Bahan-bahan (1 porsi)

• 1 tablespoon unsalted butter
• 2 tablespoons (packed) dark brown sugar
• 1/8 teaspoon vanilla extract
• Pinch salt
• 1 egg yolk
• 3 tablespoons all-purpose flour
• Pinch baking soda
• 1 heaping tablespoon semisweet chocolate chips