Heat a nonstick skillet over medium heat. (If your stove runs hot, start at medium-low.) Spread plenty of soft butter on the outside of both pieces of bread. Drop one slice of bread, buttered side down, into the hot skillet.
Cover the slice of bread with half of the shredded cheese. Place the bacon over the cheese, along with sliced jalapenos if desired. Top with the remaining cheese and the second slice of bread, butter side up.
Cover the pan with a lid – this is important, so don’t skip this part. Let the sandwich cook undisturbed for at least a minute. Uncover and check to see if the bottom slice of bread has browned. Let cook until golden brown, usually about 3 minutes on medium heat.
Press down slightly on the top slice to help the cheese stick to both slices. Use a thin spatula to carefully flip the sandwich over. Cover with a lid once more.
If your cheese isn't melting as quickly or smoothly as you’d like (especially when using pre-shredded cheeses) add a teaspoon of water to the edge of the pan and quickly cover with the lid. The steam helps melt even pre-shredded cheeses beautifully.
Check to see if the second slice of bread is golden brown. It should take about 3 minutes on medium heat. When it is ready, remove the sandwich to a plate and serve warm.