Fill pot or pan being used for frying with oil, about 1 1/2 inches. Turn stove to medium-high heat.
Take one of the divided biscuit dough chunks and flatten with hands or with a rolling pin, spreading out dough to about 4 inches in diameter. Make sure the edges get flattened too so that too much dough doesn't get bunched together at the end, making it hard to thoroughly cook the dough before it burns.
Sprinkle about 1 heaping tbsp of shredded mozzarella cheese. Spread about 8-10 pieces of mini pepperoni or pepperoni chunks. Sprinkle a few more pieces of mozzarella cheese on top of the pepperoni. Carefully close up the dough, to form a round doughnut shape. Make sure the bottom is pinched closed so that no cheese or pepperoni escapes. Repeat with remaining dough.
Carefully drop about 3 doughnuts into the fryer. When dough turns a golden brown, flip to other side to cook. When dough is evenly brown on both sides, lower heat to medium and continue to fry on both sides. The dough will need a few minutes on both sides to thoroughly cook inside, even though the outside may appear ready within 1 minute. You may want to start out with just a single doughnut to test how many minutes you need to fry on each side, cutting open the test one to check to see if the dough has cooked all the way. When donuts are finished, remove and place on a plate lined with paper towel. Immediately top with additional cheese, pressing gently down on the cheese so that it melts and sticks to the dough. The cheese will not melt all the way because the donut surface cools fast. Don't worry, there is another step to get it melted and pretty. Top with 4-5 pieces of pepperoni. Repeat with remaining donuts.
Arrange donuts on toaster oven pan and toast them for about 3 minutes on "toast" setting until cheese is bubbling and melted. Toasting them right after the frying will keep the nice, fried texture. If you don't have a toaster oven you can use your oven to heat up the cheese. Just don't let them bake too long or you will change the texture of the donuts. Serve immediately with some marinara sauce.
Notes
Substitute turkey pepperoni if you want these to be a little healthier.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.