In a medium-sized mixing bowl, combine the buckwheat flour, sugar, baking powder, baking soda, and salt. Whisk the dry ingredients together until well blended.
In a separate large bowl, whisk together the buttermilk, beaten egg, and vanilla extract until evenly combined.
Gradually add the dry flour mixture into the buttermilk mixture, stirring until a thick, smooth batter forms. Allow the batter to rest for approximately 5 minutes, or until bubbles appear and the mixture relaxes.
Preheat a griddle over medium heat and melt a portion of the butter. Drop large spoonfuls of batter onto the hot griddle, spacing them evenly. Cook each pancake for 3 to 4 minutes, or until bubbles form on the surface and the edges appear dry. Flip and cook the opposite side until golden brown, approximately 2 to 3 minutes.
Repeat with the remaining batter, adding more butter as needed. Serve the pancakes warm with a topping of fresh berries.