Combine the flour, brown sugar, cocoa powder, cinnamon and salt.
Add the butter, and mix with a fork, a pastry blender, or your fingers until the mixture forms crumbs. Set aside. (Refrigerate if you have a warm kitchen.)
To make the muffins:
Preheat oven to 375°F (190°C). Grease or line a 12-cup muffin pan.
Combine the flour, sugar, baking powder, baking soda, and salt. Make a well in the center of the mixture.
In a separate bowl, combine the yogurt, butter, eggs, and vanilla (if using). Add to the well in the flour mixture, and mix just until combined.
Transfer half of the batter to a separate bowl. Add the cocoa powder to one half of the batter, and mix until combined.
Alternate dropping the two doughs by teaspoonfuls into the prepared muffin cups. (Each will be about 2/3 full when you're done.) Top with crumb topping.
Bake the muffins 18 to 20 minutes, or until a pick inserted into the center comes out clean. Cool in the pan 10 minutes before transferring the muffins to a wire rack to cool completely.