Buckwheat Double Chocolate Cookies

Ingredients (10)

• 6 tablespoons unsalted butter
• 12 ounces bittersweet chocolate (60–70% cacao mass)*, chopped (about 2 1/4 cups), plus several chunks for the tops of the cookies
• 1/2 cup buckwheat flour
• 2 tablespoons tapioca flour**
• 3/4 teaspoon baking powder
• 2 large eggs, at room temperature***
• 1/2 cup plus 2 tablespoons cane sugar
• 1/2 teaspoon fine sea salt
• 1 teaspoon vanilla extract
• Flaky salt such as Maldon, for the tops