Preheat oven to 375 degrees.

Evenly distribute butter in each cup of a 6 cup popover pan (1/2 Tbsp per cup). Place pan in the oven for 3 minutes, until the butter has melted. Remove from the over and spray each vessel with cooking spray to prevent any sticking.

In a large bowl, beat eggs with milk and olive oil. Add in flour and salt, stir to combine. Fold in the marinara sauce until combined. Pour batter evenly into the popover pans.

Bake for 10 minutes and then reduce heat to 350 degrees. Bake for an additional 18 minutes, remove from oven and quickly stuff cubes of mozzarella into each popover. Bake for 5-10 minutes, or until popovers are puffed and golden brown. Serve with additional tomato sauce on the side.