How to Make It
Cook bacon in a large nonstick skillet over medium, turning occasionally, until crisp, about 8 minutes. Drain on a plate lined with paper towels; let cool completely, about 5 minutes. Discard drippings from skillet; wipe skillet clean.
Finely chop cooled bacon. Toss together chopped bacon, Cheddar cheese, Gouda cheese, and pepper in a medium bowl. Stir together mayonnaise and Parmesan cheese in a small bowl.
Spread mayonnaise mixture evenly on 1 side of bread slices. Arrange 6 of the bread slices, mayonnaise sides down, on a cutting board; top evenly with bacon-Cheddar mixture (1/2 packed cup per slice). Top with remaining 6 bread slices, mayonnaise sides up.
Return skillet to heat over medium-low. Add 2 assembled sandwiches to skillet; cook, partially covered, until bread is browned on both sides and cheese is melted, about 3 to 4 minutes per side. Transfer to a cutting board. Repeat process with remaining 4 sandwiches.
Cut each sandwich into 4 triangles or long sticks, and serve warm.