Preheat oven to 375F and line muffin tin with liners.
In a large bowl, combine the flour, brown sugar, cocoa powder, espresso powder, baking powder, baking soda and salt. In a separate bowl, whisk together the melted butter, buttermilk, eggs and vanilla. Pour the batter mixture into the flour mixture and fold gently to combine- being careful not to over-mix. Stir in the chocolate chips.
Using a large ice-cream scoop, divide batter into muffin liners. (Recipe yields about 21 muffins).
To make topping:
In a small bowl combine the flour, sugar and toffee bits. Cut the butter into the mixture, creating a coarse and crumbly texture. Sprinkle about a tablespoon over each muffin before baking.
Bake 18-20 minutes, until a toothpick can be inserted and is clean when removed.
Let cool on cooling rack before devouring.
Recipe by pretty plain janes at https://prettyplainjanes.com/2013/09/04/chocolate-coffee-toffee-muffins/