Cookie in a Mug

Bahan-bahan (1 porsi)

• 2 tablespoons mentega
• 4 teaspoons light brown sugar
• 4 teaspoons gula pasir
• 1/4 teaspoon ekstrak vanila
• pinch of salt, (or to taste)
• 1 egg yolk
• ¼ cup all-purpose flour
• 2 tablespoons chocolate chips, (semi-sweet or milk)