Melt butter in a microwave-safe small bowl and let cool slightly.
Mix flour, sugar, matcha powder, baking powder, and salt in a microwave-safe mug.
Stir in milk, vanilla, and the butter.
Stir in white chocolate chips.
Microwave for 75 seconds on high.
Let cool for a couple of minutes, then serve.
Notes
Use a regular ceramic mug. One with straight sides and a more narrow opening is better than one with a very wide opening. A narrow tube helps the cake develop volume.
Don’t fill your mug more than halfway.
Mix the batter well to make sure there isn’t a chunk of flour at the bottom.
Don’t worry if your cake deflates a bit after it comes out of the microwave—this is going to happen with a mug cake recipe.
Check the wattage of your microwave—you may need to adjust your cooking time more or less. This recipe was tested with a 800-watt microwave.
Clean the edges of your mug with a spatula or wipe the rim of your mug with a damp paper towel after adding all of the ingredients. This will ensure your matcha mug cake cooks and rises evenly.
Make sure to let your cake cool slightly before eating, because the middle can be extremely hot when it comes out of the microwave.