Beat butter and mayonnaise in a mixer until well blended, scraping down the sides of the bowl to ensure complete mixing. Set aside.
Prepare your favorite macaroni and cheese recipe. When finished, spread evenly into a sheet pan or casserole dish to a thickness of about 3/4-inch. Cover with plastic wrap and cool in refrigerator.
Cut the chilled macaroni and cheese into squares that are slightly smaller than the diameter of your bread.
Melt 3 tablespoons butter in a heavy-bottomed pan. Add sliced onions and cook over a medium-high heat stirring frequently until onions have caramelized. Season with salt and pepper.
Heat up BBQ sauce and add pulled pork. Allow to simmer for 5 minutes over low heat.
Generously butter one side of each slice of bread with the butter-mayonnaise mixture. Be sure to butter to the very edges!
On unbuttered side, layer as follows:
Top with slice of bread, buttered-side out.
Use a flat, nonstick griddle and bring to a medium heat, approximately 340 degrees F.
Lay sandwiches on hot griddle and proceed to cook on one side until golden brown, about 3 to 4 minutes. A bacon press or heavy pan may be placed on top of sandwiches to help with interior melting. Flip sandwich and repeat.
Remove the sandwiches from the heat and let cool for 1 minute before cutting. Enjoy!