Makes 25 to 30 sandwich cookies

For the chocolate wafers:
1 1/4 cups (155 grams) all-purpose flour
1/2 cup (45 grams, but cocoa weights can vary greatly) unsweetened Dutch process cocoa
1 teaspoon baking soda
1/4 teaspoon baking powder
1/4 teaspoon salt
1 to 1 1/2 cups (200 to 300 grams) sugar [see recipe note]
1/2 cup plus 2 tablespoons (1 1/4 sticks or 140 grams) room-temperature, unsalted butter
1 large egg

For the filling:
1/4 cup (1/2 stick or 55 grams) room-temperature, unsalted butter
1/4 cup (50 grams) vegetable shortening
2 cups (240 grams) sifted confectioners’ sugar
2 teaspoons (10 ml) vanilla extract