Preheat your oven to 350°F (175°C) and line a mini muffin pan with paper liners.
In a medium saucepan, melt the semi-sweet chocolate chips and butter over low heat, stirring constantly until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, combine the granulated sugar and eggs. Whisk until the mixture is light and fluffy.
Stir in the melted chocolate mixture and vanilla extract until well combined.
In a separate bowl, sift together the flour, cocoa powder, espresso powder, and salt. Gradually add the dry ingredients to the chocolate mixture, folding gently until just combined. Be careful not to overmix.
If using, fold in the chopped walnuts for added crunch and flavor.
Using a spoon or small cookie scoop, portion the brownie batter into the mini muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 10-12 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to keep them fudgy.
Allow the brownie bites to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Notes
Serve on a decorative platter, dusted with powdered sugar or cocoa powder, and garnish with coffee beans.