Sift together the flour and baking soda and set aside. In a mixer fitted with a whip attachment, cream the butter until light and fluffy. Then add both sugars and continue mixing to cream them together. Add the eggs gradually and let them thoroughly mix in. Add the melted unsweetened chocolate and mix in well. Add the vanilla and 1/3 of the yogurt and mix, then add half the dry ingredients, and mix in well. Repeat alternating wet and dry, ending with wet.
Pour batter into a muffin tins, lined with cupcake papers, filling them 3/4 full. Bake until firm to the touch on the cap of the cupcakes, about 20 minutes. Let cool in the pan, then decorate.
To make the cupcakes into spiders, frost with your favorite chocolate frosting glue 2 flat chocolate buttons on the top of the cupcake as eyes. Pipe a large white dot on lower half of each button then stick on the chocolate chips, then pipe a tiny white dot on the tip of the chip. In red, pipe a smile and to make them tarantulas, pipe a red hourglass shape on its back. Cut black liquorice into 3-inch long pieces and tuck 8 of them into the rim of the cupcake cap to make legs.
To serve the cupcakes, with black frosting pipe a spider web onto a middle of a large white plate and place the spiders around the edge.
Notes about the recipe: I always buy the large tub of plain yogurt because it's so much cheaper than buying all those small ones, and the next thing I know, it's gone moldy for lack of attention. So this is a good place to use a large quantity of it up. You can use sour milk instead or sour your own by squeezing a good amount of lemon juice into whole milk and letting it stand 2 minutes to curdle and sour.
Here's proof that there is something even cuter than a cupcake. Princess Diana's butler, Paul Burrell, taught me how to turn cupcakes into spiders. He made these on the Princes' birthdays, as part of a complete bug theme party with a big ladybug cake and cheese pastries coiled up like snails, with antennae made of chives. I make them for my prince's birthdays; my son Gio loves them.