Preheat oven to 400°F
Cook the macaroni in lightly salted boiling water until just before al dente, about 5 minutes. Drain, set aside.
In a large pot over medium high heat, melt the butter. Whisk in the flour and cornstarch until well combined. Add the milk and beer, bring to a low simmer, do not boil.
A hand full at a time add the cheese, whisking until all the cheese has melted before adding more.
Stir in the salt and garlic powder (add additional to taste).
Add the noodles, stir until well combined, allow the noodles to finish cooking in the sauce, about 3 minutes.
Cut the jalapenos in half, scoop out the seeds and membranes.
Place the jalapenos, cut side up, on a baking sheet covered with parchment.
Fill the jalapenos with mac n cheese.
Sprinkle with shredded cheese.
Stir together the melted butter and panko. Sprinkle the panko on top of the jalapenos.
Bake for 15-20 minutes or until the panko has browned.
Serve warm.