Bake the Book: Salt-N-Pepper Sandwich Cookies

3 1/2 cups all-purpose flour

3/4 teaspoon salt

1/4 teaspoon fleur de sel, plus more for decorating

2 teaspoons white pepper

1/4 cup dark unsweetened cocoa powder (like Valrhona)

1 1/2 cups (3 sticks) unsalted butter, cut into 1-inch cubes, cool, but not cold

1 1/4 cups granulated sugar

1 3/4 cups confectioners' sugar

3 large egg yolks

3 ounces good-quality dark chocolate (60 to 72%), melted for the vanilla filling

For the vanilla filling:

5 ounces vegetable shortening, at room temperature

2 ounces (4 tablespoons) unsalted butter, cut into small chunks, at room temperature

3 1/2 cups confectioners' sugar, sifted

1/2 teaspoon salt

1 tablespoon pure vanilla extract

1 teaspoon light rum