Preheat the oven to 375°F (190°C) and line a standard muffin tin with paper liners.
For the Muffins: In a large bowl, whisk together the flour, brown sugar, sifted cocoa powder, espresso powder, baking powder, baking soda, and sea salt.
In a separate small bowl, combine the melted butter, buttermilk, eggs, and vanilla extract. Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix. Stir in the chocolate chips.
Evenly divide the batter among the muffin cups, filling each about ¾ full.
For the Toffee Crunch Topping: In a small bowl, mix the flour, brown sugar, and toffee bits. Cut the butter into the mixture using the back of a fork until it becomes coarse and crumbly.
Sprinkle about a tablespoon of the toffee crunch topping over each muffin.
Bake for 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffin tin from the oven and let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.