Preheat the oven to 375°F (190°C).
Line a 12-cup muffin tin with paper liners or lightly grease each cup.
In a large bowl, whisk together the flour, baking powder, salt, pepper, and garlic powder.
Stir in the shredded cheese, sweetcorn, and chopped chives until evenly coated with the flour mixture.
In a separate bowl, whisk together the eggs, milk, melted butter, and Dijon mustard if using.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined. Do not overmix.
Divide the batter evenly among the muffin cups, filling each about three-quarters full.
Sprinkle a little extra cheese over the tops if desired.
Bake for 20–25 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack.
Serve warm or at room temperature.