In a microwave-safe mug, whisk together flour, sugar, baking powder, and salt.
1/4 cup all-purpose flour, 2 tbsp granulated sugar, ¼ tsp baking powder, Pinch salt
Stir in melted butter, milk, lemon juice, lemon zest, vanilla extract, and egg yolk. Mix until smooth.
1 tbsp butter, 2 tbsp milk, ½ tbsp fresh lemon juice, ½ tsp lemon zest, ¼ tsp 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!