How to Make It
Preheat the oven to 350° and line 2 large baking sheets with parchment paper.
In a medium bowl, whisk the flour, cardamom, salt and baking soda. In a large bowl, using an electric mixer, beat the butter with the sugar at medium-high speed until fluffy, 1 to 2 minutes. At medium speed, beat in the egg. Beat in the buttermilk and vanilla until just smooth, then beat in the dry ingredients. Fold in the oats.
Using a 2-tablespoon ice cream scoop, scoop 10 mounds of dough onto each baking sheet, about 2 inches apart. Bake in the lower and upper thirds of the oven for 13 minutes, until the cookies are puffy and set; shift the sheets halfway through baking. Transfer the cookies to racks to cool completely.
For each ice cream sandwich, scoop 3 tablespoons of the ice cream onto the underside of a cookie and top with another cookie. Wrap in plastic and freeze until the ice cream is just firm, about 30 minutes.