Ice Cream Sandwiches

Whisk flour, cocoa powder, baking soda, and salt in a medium bowl. Using an electric mixer, beat butter, sugar, and corn syrup in another medium bowl until light and fluffy, about 3 minutes. Add egg yolk and vanilla; beat until combined. Reduce mixer speed to low, add dry ingredients, and mix until just combined. Mix in sour cream. Form dough into a 3/4"-thick square, wrap tightly in plastic, and chill until firm, 2-3 hours. Preheat oven to 350 °F. Roll out dough between 2 sheets of parchment to about 1/8" thickness. Transfer dough (still between parchment) to a large baking sheet and chill until firm, at least 30 minutes. Remove top sheet of parchment (it will come away easily if dough is well chilled) and trim to a 14x10" rectangle. Bake cookie until dry and just set at edges, 8-10 minutes. Transfer baking sheet to a wire rack; let cookie cool. Trim cookie to a 12x9" rectangle, then cut into twelve 3" squares. Scoop ice cream into a medium bowl and mix just until softened, 2-3 minutes. Turn 6 cookies bottom side up. Dividing evenly, scoop ice cream on top and flatten slightly; top with remaining cookies, pressing down gently. Place sandwiches on a parchmentlined baking sheet and freeze until firm, at least 4 hours. Cut sandwiches in half on a diagonal to make triangles. Wrap in parchment, if desired, and freeze until ready to serve. DO AHEAD: Ice cream sandwiches can be made 1 week ahead. Keep frozen.