Cream yolks and sugar till pale yellow. Add half of the boiling water and the flour and the baking powder. Add remaining boiling water and vanilla and beat. Beat whites till stiff and fold in. Pour into a 9x13 pan lined with parchment and bake at 350 degrees for about 30 min. or till done. Cool in pan on rack.
Prepare icing: Cream butter. Add sugar, vanilla, and milk and beat till fluffy.
To assemble: When cake is cool, cut into 24 pieces. Ice each piece on all sides and coat on all sides with finely chopped peanuts. To store, keep in refrigerator (though can bring to room temperature to serve).