In a microwave-safe mug, whisk together flour, sugar, baking powder, and salt.
¼ cup all-purpose flour, 2 tablespoon granulated sugar, ¼ teaspoon baking powder, Pinch salt
Stir in melted butter, milk, lemon juice, lemon zest, vanilla extract, and egg yolk. Mix until smooth.
1 tablespoon butter, 2 tablespoon milk, ½ tablespoon fresh lemon juice, ½ teaspoon lemon zest, ¼ teaspoon vanilla extract, 1 egg yolk
Heat for 50-60 seconds until the top looks set but still slightly glossy. (Start with 50 sec and add 10-sec increments if needed.)
Let it cool for a minute before eating. Serve it with a scoop of ice cream, lemon curd, or a dusting of powdered sugar!