Preheat oven to 400°F.
Place jalapeños on baking sheet and roast about 20 minutes (until charred). Remove and place in resealable plastic bag for 10-15 minutes. Remove peppers from bag, slice in half; remove half of seeds and dice.
Fry pancetta in sauté pan until crispy. Remove from pan and drain on paper towel; set aside.
In sauté pan, melt 1 tablespoon butter and add diced onions. Cook over low heat until caramelized; remove from pan and set aside.
Place heavy whipping cream in saucepan and warm on low heat.
Melt 3 tablespoons butter in large sauté pan. Add flour and cook 2-3 minutes, stirring constantly.
Gradually whisk in warmed heavy whipping cream. Cook, stirring often, 8-10 minutes or until mixture is thickened and smooth.
Reduce heat to low. Reserve 1 cup aged cheddar. Add remaining cheddar and other cheeses to pan and stir to incorporate.
Once all cheese is melted and incorporated, add reserved pancetta, onions and jalapeños; keep warm.
Cook pasta according to package directions in salted boiling water until "al dente." Drain pasta, return to pan and stir in cheese sauce.
Pour mixture into buttered 10 x 15-inch baking dish and sprinkle with reserved aged cheddar.
Melt remaining 1 tablespoon butter. In small bowl, combine melted butter and bread crumbs; sprinkle mixture over top of macaroni. Bake 20-25 minutes at 400°F or until golden brown.