Preheat the oven to 325 degrees Fahrenheit (163 degrees Celsius). Line an 8x8 square pan with foil, leaving an overhang on two sides (you’ll use these to lift the brownies out later).

In a medium heatproof bowl, combine the butter, sugar, cocoa, espresso powder, and salt. Microwave on high for approximately 1 minute and 30 seconds. Remove from the microwave and stir with a rubber spatula until the mixture is smooth. It will look gritty, but once it cools a bit and you add the remaining ingredients, it smooths out.

Stir in the vanilla. Then add the eggs, one at a time, mixing well after each addition. The batter will become smooth and shiny – this is what you want. Slowly add in the flour and mix until it begins to disappear. Beat vigorously by hand for about 45 seconds.

Add the regular-sized chocolate chips and walnuts. Mix well. Pour into the foil-lined pan and spread evenly. Sprinkle the mini morsels on top.

Place in the oven and bake for 30 minutes. Remove from the oven and place on a wire rack to cool completely (about 45 minutes or so). Lift up the long ends of the foil and remove from the pan.