For a more pronounced nutty flavor, toast the walnuts lightly in a dry skillet before chopping. Be sure to use ripe, but firm avocados for the best texture and flavor. If you don't have buttermilk, you can make a substitute by adding 1 tablespoon of white vinegar or lemon juice to a liquid measuring cup, then filling the cup with milk to the 1/2 cup line. Let it stand for 5 minutes before using. Store leftover cupcakes in an airtight container at room temperature for up to 3 days.