Prepare sourdough batter 8-12 hours before making biscuits. Combine flour, baking powder, sugar, baking soda and salt in a mixing bowl. Cut butter into dry ingredients until mixture resembles cornmeal. Stir in milk and sourdough batter then mix well, forming a ball. Knead gently on a floured surface about 20 seconds. Roll dough out to about 1/2-inch thickness and cut into rounds. Place 1-inch apart on an ungreased baking sheet. Cover lightly and let rest for 30 minutes. Brush tops lightly with melted butter; bake at 400 degrees for 15 minutes, or until biscuits have puffed and are golden brown.
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