Preheat oven to 400°F and line a baking sheet with parchment paper. Combine 1 cup buckwheat flour with ½ cup warm water, 2 tablespoons olive oil, 1 tablespoon sesame seeds, and ½ teaspoon kosher salt. Lightly knead dough and roll it out very thinly. Cut into 2-inch squares, brush each lightly with water, and sprinkle with flaky salt. Transfer squares to prepared tray and bake until they are browned on the edges, about 12 minutes. Transfer to a rack to cool.