1. Heat the oven to 375 degrees and coat a standard 12-cup muffin tin with cooking spray. In a large bowl, whisk together the first five ingredients. In a separate bowl, lightly beat the eggs, then whisk in the milk and barbecue sauce. Combine the wet and dry ingredients and stir until the flour is moist. Mix in the mozzarella, mushrooms, onion, pepperoni, olives, and sun-dried tomatoes (or add your own mix-ins).
2. Evenly divide the mixture among the muffin wells. Bake the cupcakes until puffed and golden, about 25 minutes. Let them cool for 5 minutes before unmolding them. Warm the marinara sauce in the microwave or on the stovetop and serve it on the side.