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. Remove from pans using a slight twisting motion.