Preheat oven to 400 degrees F. Line a 12 cupcake pan with liners. (Note: Some commenters have suggested it's better to grease and bake directly in pan because the muffins will stick to the liners. It's less messy with the liners but the exterior of the batter does stick to the liners, so if you don't want that I would suggest baking directly into the muffin pan.)
Mix 1 1/2 cups flour, sugar, salt and baking powder. Add in vegetable oil, egg and 1/3 cup milk into flour mixture and stir until batter is smooth. It will be sticky. (Add in another 2 tsbp milk if desired.) Fold/stir in cheese and pepperoni. Fill muffin cups about 2/3 full.
Bake for about 18 minutes until muffins turn out golden and knife inserted comes out clean. Right before eating, top muffins with more cheese and about five slices of mini pepperoni and bake for 4-5 minutes at 350F until cheese is melted. Serve immediately.