Mix the 6 Tbsp cold butter cubes, ½ cup plus 1 Tbsp all purpose flour, ½ cup brown sugar, ⅔ cup chopped walnuts (optional), and pinch of salt with your fingers to incorporate the ingredients into small clumps. Just omit the walnuts if you don't want to add them.
Top each muffin with the streusel and bake.
For the Spiced Streusel with melted butter
This recipe will be enough for 6 muffins.
Melt the 5 Tbsp butter in the microwave for 10-20 seconds until it's almost all melted. Whisk with a fork. Then add the 1 cup plus 2 Tbsp all purpose flour, ⅓ cup brown sugar, 2 tsp cinnamon, and 1 tsp vanilla and whisk with a fork until combined.