Spray baking tray with spray. Combine flour, powder, & sugar in mixing bowl.
Add in liquid butter, milk, egg, & vanilla. Mix until well combined.
Pour half the batter into tray & top with marshmallows. Cover with the rest of batter & even out with spatula.
Bake for 25-35 minutes or until toothpick is inserted & comes out clean. (be careful not to overcook or your treats will be too dry! I recommend inserting a toothpick at 20, 25, & 30 minutes and removing from the oven as soon as the toothpick comes out clean.)