Buckwheat Double Chocolate Cookies

Bahan-bahan (30 porsi)

• 6 tablespoons unsalted butter
• 12 ounces bittersweet chocolate (60–70% cacao mass)*, chopped (about 2 ¼ cups), plus several chunks for the tops of the cookies
• ½ cup buckwheat flour
• 2 tablespoons tapioca flour**
• ¾ teaspoon baking powder
• 2 large eggs, at room temperature***
• ½ cup plus 2 tablespoons cane sugar
• ½ teaspoon fine sea salt
• 1 teaspoon vanilla extract
• Flaky salt such as Maldon, for the tops