How to Make It
Preheat the oven to 350° and position racks in the upper and lower thirds. Line 2 large baking sheets with parchment paper.
In a medium bowl, mix the flour with the cinnamon, cardamom and salt. In the bowl of a standing electric mixer fitted with the paddle, beat the butter with the sugar at medium-high speed until fluffy, about 2 minutes. Scrape down the side of the bowl. Add the egg and beat at medium speed until incorporated. Add half of the dry ingredients and beat at low speed until just incorporated. Beat in the remaining dry ingredients. Scrape the dough into a pastry bag fitted with a 1/2-inch star tip. Pipe the dough onto the baking sheets in 1 3/4-inch rosettes; you should have about 32. Refrigerate until chilled, about 20 minutes.
For cakey cookies, bake the cookies for about 14 minutes, until the tops are dry; for crispier cookies, bake for about 16 minutes, until the edges are golden. Shift the sheets from top to bottom and front to back halfway through for even baking. Let the cookies cool on the sheets for 5 minutes, then transfer to a rack to cool completely.
In a saucepan, bring 2 inches of water to a simmer. In the bowl of a standing electric mixer, combine the egg white with the sugar, water, corn syrup and vanilla. Set the bowl over (but not in) the pan of simmering water and stir constantly, until the sugar is dissolved, about 4 minutes. Transfer the bowl to the mixer and whip the mixture at high speed until firm, glossy peaks form, about 5 minutes.
Scrape the meringue into a piping bag fitted with a 1/4-inch star tip. Arrange half of the cookies flat side up. Pipe the meringue on top and close the sandwiches. Dust the cookies with confectioners' sugar.