Preheat the oven to 350 degrees F (180 degrees C). Line a baking sheet with parchment paper.
Beat butter and brown sugar together with an electric mixer in a large bowl until light and fluffy, 2 to 3 minutes. Beat in egg yolk, vanilla, and molasses until smooth.
In a separate bowl, whisk together oats, oat flour, cornstarch, baking soda, baking powder, salt, cinnamon, and nutmeg.
Fold oat mixture into butter mixture until just combined. Do not overmix.
Let dough rest for 15 to 20 minutes to allow the oat flour and oats to fully hydrate.
Scoop about 1 1/2 tablespoons dough for each cookie and roll into balls. Place cookies about 2 inches apart on the prepared baking sheet and gently flatten them to about 2 1/2 inches in diameter.
Bake in the preheated oven just until the edges are set and the centers still look slightly underbaked, 9 to 11 minutes. Let cookies cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.
For ice cream sandwiches, freeze cooled cookies until firm, 20 to 30 minutes, before assembling. Add 1 scoop, or about 1/4 cup, slightly softened ice cream between 2 cookies. Press gently, wrap each sandwich tightly, and freeze for at least 4 hours.
