Preheat the oven to 350 degrees.
Sift or combine the flour, corn starch, sea salt and baking powder in a bowl and whisk.
Cream together the butter and sugar. Then add in the egg, sour cream and vanilla and mix well.
Add the flour mix to the butter mix little by little. About 1/2 cup at a time.
Once the dough is well combined, dip your fingers in flour and begin to shape it. Make one cap for every stem and add them to a baking sheet lined with parchment paper. Use a melon baller for the caps if you want to be consistent. See photos for more on the shaping of the dough.
Bake for about 12 minutes. These will not brown on their caps so it is okay if you don't see browning.
When done, add to a cooling rack and make a small hole in the bottom of each mushroom cap. Then let cool for an hour.
Meanwhile prep the icing or chocolate. - For the icing, combine everything in a small bowl and whisk well. Dip each mushroom cap in and before they dry, sprinkle with cocoa powder. - For the chocolate topping, melt over a double broiler or a chocolate melter. You can also use the microwave but beware of overcooking that will seize the chocolate. Dip each mushroom in the chocolate and let dry. Then tap each mushroom cap in confectioners sugar to make the white spots. Once the chocolate or icing is dry, enjoy!