Preheat the oven to 350 degrees F (175 degrees C) on convection heat. Grease an 8x8-inch baking dish.
Place chocolate and butter in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes.
Pour chocolate mixture into a large bowl and stir in sugar. Cool slightly, then beat in eggs, 1 at a time. Stir in vanilla extract.
In a separate bowl, mix buckwheat flour, baking soda, and salt together. Add to chocolate mixture, stirring constantly to mix well. Pour batter into the prepared baking dish and smooth out the top.
Bake in the preheated oven until top is dry and edges have started to pull away from the sides of the pan, 20 to 25 minutes. Let cool completely before slicing.