Preheat the oven to 350°F. Grease a 9-inch round cake pan and line the bottom with parchment paper.
Beat the softened butter, granulated sugar, and brown sugar until creamy and slightly lighter in color, about 2 to 3 minutes.
Add the eggs one at a time, mixing after each addition. Beat in the vanilla and almond extract, if using.
In a separate bowl, whisk together the flour, rolled oats, baking soda, baking powder, salt, and dry cake mix.
Gradually add the dry ingredients to the butter mixture. Mix on low speed just until combined.
Gently fold in the rainbow sprinkles and mini chocolate chips.
Press the dough evenly into the prepared pan. Do not compact it too firmly.
Bake for 18 to 22 minutes, or until the edges are lightly golden and the center still looks slightly soft.
Let the cookie cake cool in the pan for 10 to 15 minutes, then transfer it to a cooling rack and cool completely.
To make the frosting, beat the softened butter until smooth. Gradually mix in the powdered sugar, then add the vanilla, salt, and 2 tablespoons of cream or milk. Beat until fluffy, adding another tablespoon of liquid if needed.
Pipe or spread the frosting over the completely cooled cookie cake. Add extra rainbow sprinkles.
Slice into wedges and serve at room temperature.
