For the cornbread muffins: Preheat the oven to 425 degrees F. Grease a regular-size muffin tin. Mix the buttermilk, vegetable oil and egg in a bowl. Add the cornmeal mix and salt and stir. The batter should be pourable. Pour the batter into the prepared muffin tin, filling the cups three-quarters full. Bake for 12 to 15 minutes. For the Cajun butter: Mix the butter, brown sugar, chili powder, oregano, cayenne, garlic powder, salt and pepper in a bowl. Spread on the corn muffins.