For the Cookies
1 1/2 cups gluten-free cake flour (or you can use an equal amount by weight of another high-quality all-purpose gluten-free flour)

1/2 teaspoon xanthan gum (omit if your blend already contains it)

1/2 cup (58 g) confectioners’ sugar

1/4 teaspoon kosher salt

3 packets (0.24 g) True Lemon lemon crystallized lemons

1/2 teaspoon finely chopped lemon zest (from 1 lemon)

8 tablespoons (112 g) unsalted butter, at room temperature

1 egg (60 g, without the shell) at room temperature, beaten

For the Filling
8 tablespoons (112 g) unsalted butter, at room temperature

1/8 teaspoon kosher salt

2 packets (0.16 g) True Lemon lemon crystallized lemons

2 cups (230 g) confectioners’ sugar
Juice of 1 lemon

2 to 3 teaspoons milk, at room temperature