Dissolve espresso powder in hot water in a 1-cup glass measuring cup, stirring with a small whisk. Cool slightly.
Whisk in oil and eggs until blended.
Place brownie mix in a large bowl; break up large lumps with the back of a spoon. Stir in espresso mixture until blended. Stir in chocolate morsels and walnuts. Spoon mixture into 38 lightly greased (1 3/4") miniature muffin cups, filling full.
Bake at 375 for 12 to 14 minutes or until tops are shiny and crusty and centers are set. Cool completely in pans on wire racks.
