Melt butter in quarter cup boiling water. Add flour and a dash of salt; stir vigorously. Cook and stir until mixture forms a ball that doesn't separate. Remove from heat and cool slightly. Add egg; beat vigorously until smooth. Stir in cheese. Drop dough onto greased baking sheet, using 1 level teaspoon dough for each puff. Bake at 200ºC for 20 minutes. Remove from oven; cool and split. Combine remaining ingredients, half teaspoon salt and dash of pepper. Fill each puff with 2 teaspoons.