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 a 14-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.
A tall, cylindrical mug with straight (not angled) sides will ensure that the cake bakes evenly. I found a 14-ounce mug to be the perfect size to allow the cake to rise up while baking without overflowing. (I filled my mug to the top with water and then transferred the water to a measuring cup to determine its capacity.) If you use a smaller mug, you may want to bake the cake on top of a plate or paper towel to catch any possible overflow.
I actually discovered that bottled lemon juice tasted stronger than fresh-squeezed and gave the mug cake a final flavor that was more lemony. That being said, try to find all-natural bottled lemon juice without preservatives or sulfites...the only ingredient in the bottle should be lemon juice!
The strength of individual microwaves can vary. The first time you make this recipe, be prepared to experiment a bit to ensure the perfect length of baking time for your particular microwave. The nice thing about this cake is that even if it is undercooked, you don't have to worry about raw eggs!
Unsalted butter will work just fine in this recipe, but I find that using salted butter adds a tiny flavor boost. If you only have unsalted butter on hand, consider adding a tiny pinch of salt to the dry ingredients.