1 1/2 cups (210 g) high-quality all-purpose gluten-free flour
3/4 teaspoon xanthan gum (omit if your blend already contains it)
2 1/2 cups (288 g) confectioner’s sugar
1/8 teaspoon kosher salt
8 tablespoons (112 g) unsalted butter, at room temperature
4 tablespoons (48 g) vegetable shortening
Scant 1 cup egg whites (from about 6 extra-large eggs – or buy those egg white only cartons)
2 tablespoons pure vanilla extract
2 tablespoons freshly squeezed lemon juice
1/2 cup heavy cream (or high fat nondairy milk)
8 ounces semisweet chocolate, roughly chopped
Preheat your oven to 350°F. Line a rimmed baking sheet with unbleached parchment paper and set it aside.
In the bowl of a stand mixer fitted with the paddle attachment, place the flour, xanthan gum, confectioner’s sugar and salt, and whisk to combine. Add the butter and shortening, and beat with the paddle attachment until combined. Add the egg whites, and then the vanilla extract and lemon juice, and mix until smooth.
Fill a pastry bag or Ziploc bag with the batter, and, with a plain round tip, about 1/4inch in diameter, pipe 1 1/2-inch sections of batter onto a parchment-lined rimmed baking sheet, spacing them 2 inches apart, as they will spread while baking. Place in the center of the preheated oven for 8 to 10 minutes, or until beginning to turn light golden brown around the edges. Let cool on the pan for about 5 minutes, then transfer to a wire rack to cool completely.
While the cookies cool, make the ganache filling. Place the chocolate in a medium-sized heatproof bowl and set it aside. In a small saucepan over medium heat, heat the cream until beginning to bubble around the edges. Pour the hot cream over the chocolate, and mix until all the chocolate is melted and the mixture is smooth and shiny. Set the chocolate aside to cool slightly, but still soft. Once the cookies are entirely cool, turn them all over, flat side up, in a single layer. Spoon a thin layer of filling onto the flat side of half the cookies. Press the flat side of the remaining cookies on top of the filling, lining them up as best you can.
Allow the filling to set. Serve at room temperature. Store leftovers (ha ha) in the refrigerator or freezer, separated by sheets of parchment paper.
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