In a separate bowl (or large measuring cup), whisk the buttermilk and milk together. Add the egg and whisk until homogeneous. Add the melted butter and whisk to combine.
Pour the wet ingredients into the dry ingredients and mix until just combined—a light hand means tender pancakes.
Heat a griddle or cast iron skillet over medium-high heat. Brush generously with oil. When the surface is sizzling, pour the batter (about 1/4 cup per pancake) onto the griddle/skillet—making sure not to overcrowded the pan. When you see bubbles on the top and bottom's browning (about 2-3 minutes), flip the pancakes. Cook the other side till is brown, about 1-2 minutes more. Re-grease skillet and repeat with remaining batter..
You can keep the pancakes warm in a 200F oven while you cook the next batch. Serve immediately, with plenty of maple syrup and a square of butter.