Preheat the oven to 400 degrees F. Butter 6 cups of a muffin tin for larger cornbread muffins or 8 cups for smaller ones.
Whisk the melted butter, milk, and egg until smooth. Add the cornbread mix and stir until smooth, then fold in the diced jalapeno. Divide among 6 or 8 muffin cups and top each with a jalapeno slice. Bake until golden brown, about 18 minutes.
Meanwhile, heat a large cast iron skillet over high heat. Add the beef and spread in an even layer. Cook, stirring and breaking the meat into bits, until browned, about 7 minutes. Add the onion and pepper cook, stirring occasionally, until crisp-tender, about 5 minutes.
Add the chili powder and cook, stirring, for 1 minute. Add the beans and 1 cup water. Boil, mashing about half of the beans into the mixture, until the liquid is very thick, about 3 minutes.
Unmold the cornbread and split each in half. Divide the chili among the bottom halves and top with the muffin tops. Serve hot.