Combine the dry ingredients—flour, sugar, baking powder, salt, and lemon zest—in a microwave-safe mug. Stir until evenly mixed using a spoon or fork.
4 tbsp all-purpose flour, 2 tbsp granulated sugar, 1 tsp baking powder, 1 pinch salt, 1 tsp lemon zest
Add the milk, vegetable oil, vanilla extract, and lemon juice to the mug. Mix thoroughly until a smooth batter forms, with no lumps remaining. The mixture should be slightly thick but pourable.
3 tbsp milk, 1 tbsp vegetable oil, 1/2 tsp vanilla extract, 1 tbsp fresh lemon juice
Place the mug in the microwave and cook on high for about 2.5 to 3 minutes. Stop when the cake has risen and is just set in the center, with a lightly springy top. You should hear a slight ping or bubbling sound, and the batter should no longer be liquid.
Remove the mug carefully from the microwave and allow the cake to rest for 1 minute. It will slightly deflate and firm up as it cools.
Optionally, garnish the mug cake with a sprinkle of lemon zest or a dusting of powdered sugar for presentation. Serve directly in the mug for a warm, bright lemon treat.