Preheat the oven to 375°F. Line a muffin tin with paper muffin cups.
For the muffins:
In a large bowl combine the flour, brown sugar, sifted cocoa powder, espresso powder, baking powder, baking soda, and salt.
In a small bowl combine the melted butter, buttermilk, eggs, and vanilla. Pour the butter mixture into the flour mixture and stir until combined. Stir in the chocolate chips. Do not overmix.
Divide the batter evenly among the muffin cups.
For the toffee crunch topping:
In a small bowl combine the flour, sugar, and toffee bits. Cut the butter into the mixture using the back of a fork until it is coarse and crumbly. Sprinkle about a tablespoon of the mixture over each unbaked muffin.
Bake for 20 minutes, or until a cake tester inserted into the center comes out clean. Transfer the pan to a wire rack to cool for 5 minutes before removing the muffins to the rack to cool completely.
*This can be found at many grocery stores and gourmet food stores. If you can't find it you can also use dark roast instant coffee.