Makes 50 Sandwich Cookies
2 cups all-purpose flour
1 cup cornstarch
1/3 teaspoon salt
3 sticks unsalted butter, plus 3 tablespoons, softened
3/4 cup powdered sugar
1 tablespoon charcoal powder, plus more
1½ teaspoons vanilla extract
black sesame seeds, to garnish
2/3 cup heavy cream
3 tablespoons black sesame paste
6 ounces white chocolate
Preheat oven to 350F. Line 2 large baking sheets with parchment paper.
Whisk together flour, cornstarch, and salt. Beat together 3 sticks butter and powdered sugar with an electric mixer until pale and fluffy, then beat in charcoal and vanilla extract. Add more charcoal, a teaspoon at a time if you desire a darker shade. Charcoal can add be gritty if you add too much, so go slow. At a low speed, mix in flour mixture just until a soft dough forms.
Put black sesame seeds in bowl, roll a teaspoon sized dough ball into the seeds to coat, transfer to baking sheet. Bake until tops are slightly cracked, 12-15 minutes. Transfer cookies to parchment on a rack to cool completely.
Bring heavy cream and black sesame paste to a boil over medium-high heat in a saucepan. Pour over the chocolate and let sit for 2 minutes to melt. Add 3 tablespoons butter and whisk until smooth. Leave to chill in the refrigerator 1-2 hours until set. Pipe filling onto the flat side of half of the cookies, sandwich with remaining cookies.