Grease muffin forms with butter or line them with parchment paper.
In a medium bowl mix flour, cocoa powder, baking powder, baking soda and salt.
In a large bowl mix butter and sugar using a mixer on medium speed.
Add egg and vanilla and mix well.
Incorporate flour mixture and add yogurt.
Fill muffin molds with the mixture, up to two thirds.
Let the muffins bake for 16-20 minutes (test with a toothpick to see if they're done).
Remove them from the oven and let them cool in the molds for 5 minutes, then move them to a rack and let them cool to room temperature.
For decorations: Put egg whites in a bowl and mix with a mixer at high speed until you get a soft foam. Add powdered sugar, vanilla sugar and orange juice, and mix until you get a thick, glossy composition (If it's too soft, add more powdered sugar, if too thick add orange juice). Move 1/3 of the mixture in a small bowl and add food coloring or cocoa powder. Put the colored mixture in a decorative cloth or plastic bag or syringe. Apply the icing with a spoon over muffins and spread it with a knife. Draw lines using colored (I made spider webs on some and spider legs and head on others). Cut Ferrero Rocher bonbons in half and use as spider body
You can decorate with M&M's, cereals or raisins
White icing can be replaced with melted white chocolate
Add walnuts, nuts, raisins, fresh fruit into the muffins composition if preferred