Whisk the buckwheat flour, all-purpose flour, sugar, baking powder, baking soda and salt in a large bowl.
In another bowl, whisk the buttermilk, eggs, melted butter and vanilla.
Pour the wet ingredients into the dry ones and stir just until no streaks of flour remain. A few lumps are fine. Let the batter rest for 10 minutes; it will thicken and turn slightly bubbly.
Heat a griddle or large skillet over medium heat and brush lightly with butter.
Pour 1/4 cup of batter per pancake. Cook for 2–3 minutes, until bubbles form on the surface and the edges look set, then flip and cook for 1–2 minutes more.
Keep cooked pancakes warm on a tray in a 200°F (95°C) oven while you cook the rest.