In a small saucepan, melt the butter over medium-low heat. Continue stirring until it turns brown and nutty in aroma. Remove from heat and cool for 5 minutes.
Add the flour, brown sugar, and nuts, and stir with a fork until it resembles clumps of sand.
Spoon about a tablespoon of the streusel on top of each muffin, making sure that the batter is covered.
Bake for 20 minutes or until toothpick inserted in the center comes out clean. Cool down.