Preheat oven to 400 degrees Fahrenheit. 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 together the wet ingredients (melted butter, egg, milk) until well combined. Make a well in the center of the flour mixture and add the egg mixture, stirring only until just combined.
Combine all the topping/filling ingredients. Gently fold half of the mixture into the muffin batter.
Spoon the muffin batter, about ¼ cup, into each muffin tin. Top with remaining brown sugar mixture. Bake at 400 degrees for 12 to 15 minutes, or until muffins spring back when pressed lightly. Remove from oven. Cool slightly, then transfer carefully to a wire rack.
Whisk together all of the frosting ingredients. Drizzle over the top of the muffins and enjoy every single delicious bite . . . maybe even lick your fingers when you’re done.