Scoop the ice cream 3 scoops at a time into a large piping bag and use your hands to warm the ice cream slightly. Fill a taco shell with ice cream almost to the top, then squeeze the shell a bit to compress the ice cream and push it even with the rim of the shell. Put the filled shell back in the freezer, then continue with the remaining taco shells and ice cream, refilling the piping bag as needed. Freeze until the ice cream is solid again, about 30 minutes.
Melt the chocolate and 1/2 cup of the cream in a double boiler. Stir until smooth and shiny. If it is too thick (depending on the chocolate brand), add a bit more cream--the chocolate mixture should cling to the taco when dipped, but not run right off. Dip the top of each taco in the chocolate. Fill in any empty spots by spooning on additional chocolate and spreading it. Let any excess drip off, then roll each in the chopped pepitas while the chocolate is still setting. Freeze again to set the chocolate, about 30 minutes.