Cook bacon in a skillet over medium heat for 8 to 10 minutes until browned and crisp. Transfer to a plate and leave only 1 tablespoon rendered fat in the skillet.
Cook onion in the reserved fat for 12 to 15 minutes until soft and golden. Add bourbon, vinegar, brown sugar, water and pepper and scrape the bottom.
Return bacon and simmer for 8 to 10 minutes, stirring often, until the mixture is thick and spreadable with little free liquid. Transfer to a bowl and cool for 5 minutes.
Divide half the Gouda over 4 bread slices. Spread the bacon jam evenly over them, add remaining Gouda and close with the other bread slices.
Butter the outside faces. Cook 2 sandwiches at a time in a large skillet over medium-low heat for 3 to 4 minutes per side until golden and the Gouda is melted, repeating for the second batch.
Rest each batch for 1 minute before cutting and serve one sandwich per person.