In a small bowl, whisk together flour, sugar, and baking powder until thoroughly, completely combined, with no streaks of any ingredients remaining.
Blend in milk, lemon juice, butter, vanilla, and poppy seeds until batter is smooth.
Pour batter into a14-ounce (or larger) microwave-safe mug with straight sides.
Microwave on high for 1 1/2 to 2 minutes, or until cake is done to your liking.
Allow to cool for a couple of minutes before serving.