Preheat the oven to 400°F (200°C) and generously grease a standard 12-cup muffin tin.

In a large bowl, sift together the dry ingredients: flour, baking powder, salt, and sugar.
In a separate bowl, whisk the wet ingredients: melted butter, egg, and milk until smooth.
Make a well in the center of the flour mixture and pour in the egg mixture. Stir gently until just combined don’t overmix.

In a small bowl, combine all the topping/filling ingredients. Fold half of this mixture into the batter.

Spoon the batter (a little less than ¼ cup each) into the muffin cups. Sprinkle the tops with the remaining brown sugar mixture.

Bake for 12–15 minutes, or until the muffins spring back lightly when pressed. Remove from the oven, let cool slightly, then transfer to a wire rack.

Finally, whisk together the frosting ingredients and drizzle over the warm muffins. Enjoy every bite finger-licking encouraged!