Cream the butter and sugar together, then blend in the egg yolk.
Stir in the flour, baking powder, and salt until a soft dough forms.
Chill the dough, wrapped, for 1 hour.
Heat the oven to 350°F and line a baking sheet.
Roll dough to 1/4 inch thick and cut into rounds.
Bake 8-10 minutes until edges turn golden, then cool fully.
Whisk powdered sugar with egg white or meringue powder mixture, thinning with water to a spreadable icing.
Spread icing over each cookie and let it dry completely.
Pipe a black wavy line on each cookie and press on M&M's as bulbs. Let set before serving.