▢4 medium plums, halved and pitted ▢¾ cup sugar ▢½ cup creme fraiche ▢½ cup olive oil ▢1 tablespoon lemon zest ▢2 large eggs ▢½ cup buckwheat flour ▢½ cup all purpose flour ▢1 teaspoon baking powder ▢½ teaspoon baking soda ▢½ teaspoon cardamom ▢½ teaspoon salt ▢2 tablespoons demerara sugar ▢2 tablespoons almond flakes ▢confectioners' sugar for serving (optional)