Preheat oven to 400 degrees F. Line a 12-cup muffin pan with liners; coat with nonstick spray.
For the streusel, combine 1/2 cup flour, 1/3 cup sugar, 1/4 cup ground pretzels and 1/4 tsp. cinnamon. Stir 6 Tbsp. butter into dry mixture, cover and chill.
For the muffins, whisk together 1 cup flour, 1 cup ground pretzels, chocolate chips, baking powder, salt and 1/4 tsp. cinnamon. Whisk together milk and 2/3 cup sugar until sugar dissolves. Add eggs, oil, 4 Tbsp. butter and vanilla; stir into flour mixture. Let batter sit 2 minutes, then divide among muffin cups.
Squeeze streusel into clumps; top muffins with it and sprinkle with coarse salt. Bake muffins until a toothpick inserted in centers comes out clean, 17-19 minutes. Let muffins cool in pan 10 minutes.
~Adapted from Cuisine at Home