Place softened butter and sugar in a medium sized bowl and mix on medium/high speed with a mixer until combined and creamy.
Then, add the vanilla and eggs, mix again to combine.
Whisk together the dry ingredients: flour, cornstarch, baking powder, baking soda, and salt. In 2 increments, add the dry ingredients to the cookie batter and mix slowly until just combined.
Add in the sprinkles (not nonpareils) and the white chocolate chips. Mix slowly with a spatula until combined.
Spread the batter into a well greased pie dish or cake pan. Place in the freezer to chill for 10 minutes while the oven preheats to 350 degrees F.
Bake for 25 to 30 minutes, or until the top is slightly browned. Allow the cookie cake to cool completely before frosting.
For the frosting:
Combine softened butter with powdered sugar and beat on high until fluffy, start at low speed first so you don't get powdered sugar all over the place.
Add in the vanilla, almond extract (optional), heavy cream and salt. Beat on high until well combined and fluffy, about two minutes.
Place the frosting in a piping bag with a Wilton 1M piping tip and pipe along the edge to create a pretty border. Sprinkle some of the confetti sprinkles on top.
Notes
Keep the cookie cake refrigerated, covered in plastic wrap, for up to five days. Remove from the fridge about 30 minutes before serving so it comes to room temperature.