For a milder muffin, remove the seeds from the jalapeños before dicing.
Preheat oven to 400°. Whisk together first 7 ingredients in a bowl.
Place 4 tablespoons butter in a medium microwave-safe bowl. Microwave at HIGH 30 seconds or until butter melts. Whisk buttermilk and egg into melted butter. Add buttermilk mixture to cornmeal mixture, stirring until just combined.
Grease 8 muffin cups with remaining 1 tablespoon butter. Spoon batter evenly into prepared muffin cups. Bake 16 minutes or until edges are lightly browned. Let cool in pan on a wire rack 5 minutes. Serve warm or at room temperature.