Place the bacon slices in a cold cast iron skillet and turn the heat to medium, cooking until the fat renders and the bacon becomes perfectly crispy and golden.
Remove the bacon to a paper towel-lined plate to drain, then pour off the excess fat from the skillet, leaving just a light coating.
Divide the ground beef into four small, loose balls (about 1.25 ounces each) and season the outside of each ball with sea salt, black pepper, and garlic powder.
Increase the skillet heat to medium-high and place the beef balls in the pan, immediately using a heavy spatula to smash them down into very thin patties.
Sear the patties for about 90 seconds until a deep brown crust forms, then flip them over and top two of the patties with the sharp cheddar cheese.
Stack the remaining two plain patties on top of the cheesy ones to create two double-stacked portions, then cover the pan for 30 seconds to finish melting the cheese.
Lightly toast the slider buns cut-side down in the skillet for 30 seconds until the edges are slightly crisp and warm.
Assemble the sliders by spreading mustard on the bottom buns, followed by the butter lettuce, tomato slices, the double-stacked cheesy patties, and the crispy bacon.