Stir together flour, cocoa powder, baking powder, and salt in a small bowl or mug.
Add peanut butter, Greek yogurt, maple syrup, vanilla extract, and milk. Mix well until evenly combined - it should be smooth and thicken a bit.
Stir in chocolate chips.
Microwave on high for 45 seconds. Check and microwave for about 15 seconds more, until the top is set and it's just a little gooey in the center.
For a s'mores cake, add 2-3 marshmallows during the last 15 seconds of cooking time. Top with crushed graham crackers.
Other delicious variations...
-Top it with sliced strawberries and a scoop of vanilla ice cream
-Drizzle on caramel and add a sprinkle of sea salt
-Try it with a raspberry compote and a sprinkle of powdered sugar
Serving Size 1 mug cake