In a small bowl or 2-cup measuring cup, beat the egg until foamy. Add buttermilk and vanilla and blend well. Add the baking soda, one teaspoonful at a time, sprinkling it in and beating until the mixture is smooth and the consistency of light cream. Add the lemon juice all at once and blend into the mixture. Stir, do not beat (you want it creamy but without a lot of air). The mixture will congeal into a pasty lump. Scoop it out of the bowl using a spatula and spread it on a floured surface. Sift the flour and 3/4 cup sugar together and use the fingertips to work it into the egg-lemon mixture. With a floured rolling pin, roll the dough out 1/32 inches thick, and with the tip of a sharp knife, cut the 'angel' shapes and twist up the edges to form a shell-like curve about 3/8 inch high. Sprinkle on the remainder of the sugar. Brush each angel with melted butter. Place angels one inch apart on an ungreased baking sheet and bake for 12 minutes or until golden.
Recipe courtesy of "Penn and Teller's How to Play with Your Food" by Penn Jillette