Place sliced cheese (from block) or shredded cheese in a bowl close to the stove.
Place 2 tablespoons of butter (salted or unsalted) in a skillet over medium heat on top of the stove. This is for 2 slices. Use more or less as preferred.
Use a spatula to slide bread around skillet to absorb melted butter. Flip over and repeat until second side is also coated.
Leave one side face down to brown over medium heat (30 to 35 seconds).
QUICKLY arrange pepperoni pieces in a single layer on bottom of pan (opposite side of cheese toast). After about 5 seconds, remove pieces and drain on folded paper towel.
Flip bread over. Immediately reduce heat to low. Quickly arrange cheddar cheese on top of "toasted" side.
HELPFUL TIP: If cheese doesn't completely melt, use a spatula to flip over into the HOT skillet for 2 to 3 seconds. After flipping over, immediately turn heat OFF.
Remove toast from skillet. Arrange six (or more) hot pepperoni pieces on top of each slice. Sprinkle with red pepper flakes (optional).
Serve warm or hot.