Sweetened whipped cream and cinnamon, for serving (optional)
Preheat oven to 350 degrees. In a mixing bowl, whisk together flour, oats, salt and baking soda for 30 seconds. Add in 1/2 cup granulated sugar and 1/2 cup brown sugar an mix until no clumps remain.
Combine melted butter and vanilla and add to mixture, then stir with a spoon until evenly moistened.
Gently press half of the mixture into a greased 8 by 8-inch baking dish (a 9 by 9-inch baking dish would be great to, just reduce the baking time slightly as needed) and bake in preheated oven 15 minutes.
Meanwhile in a mixing bowl, whisk together 1/4 cup granulated sugar, 1/4 cup brown sugar, cinnamon, nutmeg, ginger, cloves and salt.
Add in egg, egg yolk and vanilla and stir until blended. Mix in pumpkin then milk.
Pour mixture over baked cookie portion and return to oven to bake 15 minutes, then remove from oven and sprinkle remaining cookie crumb mixture over top while breaking the crumb into small bits.
Transfer oven rack closer to the top-center (not directly beneath but a few levels below) and return to oven to bake about 20 - 25 minutes longer until golden on top and center only jiggles slightly.
Remove from oven and allow to cool 1 hour at room temperature, then transfer to refrigerator to cool 1 hour longer (or serve warm from the oven like a crumble with ice cream).
Cut into squares and serve with sweetened whipped cream and a light dusting of cinnamon if desired.
Nutrition estimate based on 12 servings, does not include whipped cream topping or decoration.