Cream butter; gradually add sugar, beating well. Add egg yolks, one at a time, beating well after each addition.
Combine boiling water and cocoa; stir until cocoa dissolves. Let stand 5 minutes. Stir in soda. Add cocoa mixture to creamed mixture, beating well.
Sift together flour, baking powder, salt, and spices; add to creamed mixture alternately with milk, beginning and ending with flour mixture. Beat egg whites (at room temperature) until stiff peaks form; fold into creamed mixture.
Spoon into well-greased muffin pans, filling half full. Bake at 350° for 15 to 20 minutes. Remove from muffin pans; spoon glaze over tops.