Preheat the oven to 350°F. Line 8 jumbo or 12 standard muffin pan cups with paper liners.
In a large bowl, combine the flour, sugar, cocoa, baking powder, baking soda, and salt and whisk to blend.
In a medium bowl, combine the buttermilk, eggs, and butter. Add to the flour mixture and stir just until blended. Stir in the chocolate chips.
Spoon the batter into the prepared muffin cups, filling them about ¾ full.
Make the streusel: In a small bowl, mix the flour, sugar and nuts. Add the butter and mix with a fork or your fingers until crumbly. Spoon about 1 Tbs. of the streusel over each muffin, patting gently.
Bake the muffins until set, about 30 to 35 minutes for jumbo, 20 to 25 for standard. Remove to a wire rack and let cool for 5 minutes, before removing to the rack to cool completely.