Preheat the oven to 350°F (175°C). Line a standard muffin tin with paper liners to facilitate easy removal and presentation.
In a medium mixing bowl, combine the all-purpose flour, unsweetened cocoa powder, instant coffee granules, baking soda, and salt. Whisk together until the ingredients are evenly distributed.
In a separate large mixing bowl, using an electric mixer or stand mixer, cream the softened unsalted butter and granulated sugar together until the mixture is light and fluffy. This process typically takes approximately 2 to 3 minutes.
Add the eggs to the creamed mixture, one at a time, ensuring that each egg is fully incorporated before adding the next. Stir in the vanilla extract until thoroughly combined.
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Begin and end with the dry mixture, mixing gently after each addition to ensure the batter is smooth and uniform. Avoid overmixing to maintain a tender crumb.
Gently fold the semi-sweet chocolate chips into the batter using a spatula, ensuring an even distribution throughout.
Using a spoon or ice cream scoop, portion the batter into the prepared muffin cups, filling each approximately three-quarters full.
Sprinkle the toffee bits and brown sugar evenly over the top of each muffin to create a crunchy, caramelized topping.
Bake for approximately 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs.
Allow the muffins to cool in the tin for 5 minutes before carefully transferring them to a wire rack to cool completely.