In a saucepan over medium heat, combine milk, sugar, and ground cinnamon. Stir until warm.

1. In a separate bowl, whisk egg yolks. Slowly add the warm milk mixture to temper, then return to saucepan. Cook until slightly thickened. Remove from heat, stir in vanilla, then let cool.
2. Add heavy cream, refrigerate until chilled, and churn in an ice cream maker according to manufacturer’s instructions. Freeze until firm.
3. Preheat oven to 350°F (175°C). In a bowl, whisk oats, flour, baking soda, salt, and cinnamon.
4. In another bowl, beat softened butter, brown sugar, and granulated sugar until creamy. Add eggs and vanilla, and beat until combined. Gradually mix in dry ingredients.
5. Scoop dough onto a baking sheet and bake 10–12 minutes or until golden. Cool completely.
6. Let the cinnamon ice cream soften slightly. Place a scoop between two oatmeal cookies, gently pressing them together. Smooth edges with a knife if desired.
7. Freeze sandwiches until firm before serving.