For Streusel, mash together ingredients in a small bowl until it is in small crumbs. Set aside until later. Also, preheat oven to 350 degrees.
Mix together dry ingredients in a large bowl for batter. Mix together wet ingredients in a separate bowl including melted butter.
Stir together wet and dry ingredients to form a thick batter.
Butter muffin tins and spoon batter into each tin. Fill each tin ⅓-½ full. You should fill 10 tins and use about half of the batter.
Pack a spoonful of streusel onto each muffin. You should use half of the streusel.
Top streusel with the rest of the batter. Then add the final ½ of the streusel to the muffins. It’s okay if they overflow a bit out of the muffin tins.
Bake muffins for 15-20 minutes at 350 degrees until they a tester comes out dry.
For icing, whisk together milk and powdered sugar.
Let muffins cool for a few minutes and then remove them from tin. Let them cool for another 10 minutes and then drizzle on icing.
Serve immediately or store in an airtight container.