1. Whisk together flour, baking powder, and 3/4 tsp salt in a bowl, then blend in cold butter (3/4 stick) with your fingertips or a pastry blender until it resembles coarse meal. Add milk, stirring until mixture just forms a dough, then gather into a ball.
2. Divide dough in half and roll out one piece on a well-floured counter into a 12-inch round (1/8 inch thick). Lift the dough and carefully place it in the pie plate, pushing down the edges. Pat the dough in with your fingers trim any overhang.
3. Preheat oven to 400°F with rack in middle. Whisk together mayonnaise and lemon juice.
5. Roll out remaining piece of dough into a 12-inch and place it over the filling, pressing down the edges with a fork. Cut 4 steam vents in top crust and brush crust with melted butter (2 teaspoons). Bake pie until crust is golden and filling is bubbling, 30 to 35 minutes, then cool on a rack. Serve warm or at room temperature.