In large pot of boiling salted water, cook pasta according to package instructions. Drain well and set aside.
Melt 1 tablespoon butter in medium saucepan over medium heat. Whisk in flour until well combined. Gradually whisk in milk and cook, whisking constantly, until incorporated, 1-2 minutes. Stir in cream cheese, then gradually whisk in cheddar, asiago and romano, a handful at a time, until smooth, about 1-2 minutes. Remove from heat. Add cheese sauce to pasta and stir.
Heat remaining butter in grill pan over medium-high heat. Spread mayonnaise evenly on one side of each slice of bread. Place 2 slices pancetta on one side of sourdough bread (without mayonnaise) and top with generous scoop of prepared macaroni and cheese; top with another slice of bread (mayonnaise facing out) to create sandwich. Repeat with remaining slices of bread and macaroni and cheese to make six sandwiches.
Place sandwich in pan and grill, flipping once, until bread is golden and filling is heated through, about 2-3 minutes per side.