Spread mayonnaise on both slices of bread; set aside. Heat a small skillet over medium heat. Melt half of butter, then place one slice of bread, mayonnaise side down, into pan. Layer with sliced cheese, bacon, mac and cheese and jalapeños if using. Top with second slice of bread, mayonnaise side up. Cook until bottom piece of bread is golden brown, about 4-5 minutes. Lift sandwich and add remaining butter. Flip sandwich and gently press down sandwich for even cooking. Cook until second side is golden brown. Remove from heat. Serve immediately.
To make Easy Mac and Cheese
Preheat the oven to 350°F. Cover a 9×13-inch baking dish with non-stick spray.
In a small saucepan over medium heat, bring milk to a slight simmer (tiny bubbles will appear at edge of pan). Remove from heat.
In second saucepan over medium heat melt butter over medium. Add the flour and stir to combine, making sure to scrap bottom of pan to prevent any scorching. Cook until mixture starts to just turn brown color, about 3 minutes. Gradually add in milk and stir constantly until sauce thickens, about 3 minutes. Remove from the heat and stir in the dry mustard, salt, and pepper. Add the cheese and stir until completely melted. Salt and pepper to taste. Add elbow macaroni and stir to coat.
Transfer mixture to prepared baking dish. Bake until top starts brown and bubble, about 30 minutes.