Preheat oven and butter pan:
Heat oven to 375°F (190°C). Butter a muffin pan.Mix batter and fold cheese:
In a bowl, mix flour, baking powder, and salt. In another bowl, whisk milk, egg, and melted butter. Slowly add wet to dry, stirring until just blended. Fold in half the mozzarella.Layer batter, sauce, and pepperoni:
Drop a thin layer of batter into each muffin cup, filling halfway. Top with 1 tsp marinara and a few pepperoni slices. Cover with more batter to 3/4 full. Sprinkle remaining mozzarella and add a few pepperoni slices on top.Bake and garnish with basil:
Bake 15-20 minutes until golden and a toothpick inserted in center comes out clean. Cool slightly in pan, then move to a rack. Garnish with fresh basil. Serve warm.