For the cookies:
- 2 cups plus 2 tablespoons all-purpose flour
- 1/2 cup unsweetened Dutch-process cocoa powder
- 1/4 cup plain malted-milk powder
- 1 teaspoon baking soda
- 1/2 teaspoon coarse salt
- 1 cup (2 sticks) unsalted butter, softened
- 1 3/4 cup sugar
- 1 large egg
- 2 teaspoons vanilla extract
- 1/4 creme fraiche (or sour cream)
For the filling:
- 10 ounces semi-sweet chocolate, coarsely chopped
- 4 tablespoons unsalted butter, cut into small pieces
- 1 cup malted-milk powder
- 3 ounces cream cheese, room temperature
- 1/4 cup plus 2 tablespoons half-and-half
- 1 teaspoon pure vanilla extract
Preheat oven to 350 degrees F. Sift flour, cocoa powder, malted-milk powder, baking soda and salt.
Mix butter and sugar on medium speed until pale and fluffy. Mix in egg, vanilla and creme fraiche (or sour cream) and 3 tablespoons hot water. Reduce speed to low and mix in flour mixture.
Space tablespoon-size balls of dough 3.5 inches apart on parchment-lined baking sheets. Bake until flat and just firm. 10 to 12 minutes. Let cool on parchment on wire racks.
Melt chocolate and butter in a heatproof bowl set over a pan of simmering water, stirring. Let cool. Mix malted-milk powder and cream cheese on medium speed until smooth. Gradually mix in half-and-half, chocolate mixture and vanilla. Refrigerate, covered, until thick, about 30 mins. Mix on high speed until fluffy, about 3 minutes.
Assemble cookies: Spread a heaping tablespoon filling on the bottom of 1 cookie. Sandwich with another cookie. Repeat. Sandwiches can be refrigerated between layers of parchment in an air-tight container up to 3 days.