I remember getting a treat like this when I was pre-kindergarten (my Mom's ride-share gave it to me on the way to the babysitter's). And, while the brownie was dry, I remember being fond of the memory, so when I found a version in a Taste of Home magazine, I tweaked it with my favorite brownie batter, and this is what we have. It still needs a better cook time (as evidenced by the hole in my cone in the picture)
Beat sugar, eggs, and cocoa. Add flour, butter, walnuts, and vanilla.
Place the ice cream cones in muffin cups; fill cone barrel with batter. Bake at 350*F for a minimum of 35 minutes or until brownies are set on top and a toothpick inserted near the center comes out with moist crumbs (do not overbake).
Cool completely. Just before serving, top each with a scoop of ice cream and garnish with sprinkles.