Preheat oven to 325 degrees. Line an 8x8 square pan with foil, leaving an overhang on two sides (you will pull them out of the pan this way).
In a medium heatproof bowl, combine butter, sugar, cocoa, espresso powder and salt. Microwave on high for approximately 1 minute and 30 seconds. Remove from microwave and stir with a spatula until mixture is smooth. It will look gritty but once it cools a bit and add the remaining ingredients, it smoothes out.
Stir in vanilla. Then add 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 flour, mix until it begins to disappear. Beat vigorously by hand for 45 seconds or so.
Add regular sized chocolate chips and walnuts. Mix well. Pour into foil-lined pan, spread evenly. Sprinkle mini morsels on top.
Place in oven and bake for 30 minutes. Remove from oven and place on top of a wire rack to cool completely (about 45 minutes or so). Lift up the long ends of the foil and remove from pan.