Preparation Whisk flour, sugar, baking powder, baking soda, and salt in a large bowl. Using a fork, beat eggs, buttermilk, seltzer, vanilla, and 3 Tbsp. melted butter in a medium bowl to incorporate eggs. Add egg mixture to dry ingredients and whisk to combine (batter will be slightly lumpy). Heat a large griddle or 2 cast-iron or nonstick skillets over medium; brush with butter. Working in batches, scoop 1/2-cupfuls of batter onto griddle; cook until bubbles form on the surface and pop and the underside is golden brown, 2 1/2–3 minutes. Flip and continue to cook until golden brown on the bottom, 2–2 1/2 minutes more. Transfer pancakes to plates or a platter and brush tops with butter. Serve with maple syrup alongside. Cooks' NoteTo feed a larger group, double the recipe and keep pancakes warm in a 250°F oven between batches. If you don't have a griddle or 2 skillets, use 1 skillet to cook the pancakes.