Preheat oven to 350°F. Line a 9-inch cake pan with parchment paper. Spray the sides of the pan with cooking spray.

Place the butter, granulated sugar, and brown sugar in the bowl of a stand mixer. Beat on high until fluffy. Add in the egg, egg yolk, and vanilla; beat on low until combined. Scrape the sides of the mixing bowl to make sure all of the ingredients are incorporated evenly.

In a large bowl, sift together the flour, cornstarch, baking soda, and salt. Add half of the flour mixture to the mixing bowl; beat on low just until combined. Add the remaining flour mixture, and beat on low just until combined. Stir in the pumpkin spice morsels and semi sweet morsels.

Spread the dough in an even layer in the prepared cake pan. Top with additional pumpkin spice morsels. Bake for 25–28 minutes or until the top of the cookie cake is golden brown. Allow to cool completely before frosting.

Place the butter and brown sugar in the bowl of a stand mixer. Beat on high until fluffy. Add the maple syrup, milk, and sea salt; beat until combined. Slowly add the powdered sugar, ½ cup at a time, beating in between each addition. Add a splash more milk if desired.

Spoon buttercream into a piping bag fitted with a star tip. Pipe the salted maple buttercream on the cooled cookie cake. Decorate with additional pumpkin spice morsels and semi sweet morsels. Serve immediately. Can also be refrigerated up to 3 days.