1. mix the lemon into the milk. set aside and allow to curdle. about 10 minutes. i've found that it's easier to curdle milk when it's warm. after milk has separated into little curds, immediately stick into freezer as the milk mixture should be very cold before mixing with butter.
2. adjust oven rack to middle position and heat oven to 450 degrees.
3. whisk flour, baking powder, baking soda, sugar, and salt in large bowl.
4. add melted butter to milk mixture, quickly whisking until butter forms small clumps.
5. add milk mixture to dry ingredients and stir with wooden spoon until just incorporated and batter pulls away from sides of bowl forming a sticky ball. if batter feels too firm and dry bits are not gathering into a ball, sprinkle on more milk, about another tablespoon. be careful not to overmix, as that will give you a less fluffy biscuit.
6. spoon batter into approximately 1/4 cup portions and drop onto parchment-lined rimmed baking sheet, about 1 1/2 inches apart. bake until tops just begin to brown and become crisp, 12 to 14 minutes.
7. brush biscuit tops with melted butter, if you like. transfer to wire rack to cool.
