Double-Buckwheat Double-Chocolate Cookies

Ingredients

• 1 2/3 cups all-purpose flour
• 1 cup buckwheat flour
• 1/4 cup unsweetened cocoa powder
• 2 sticks unsalted butter
• 1/2 cup sugar
• 1/4 cup packed light brown sugar
• 1 teaspoon fine sea salt
• 2 large egg yolks
• 1 teaspoon pure vanilla extract
• 1/4 cup kasha, preferably Wolff's medium granulation
• 4 ounces bittersweet chocolate
• 2 teaspoons sanding sugar