Preheat the oven to 350°F. Grease a 12-inch round pizza pan with sides and line the bottom with a round of parchment paper.
In a large bowl, whisk the melted butter, brown sugar, and granulated sugar for 1 minute until smooth. Whisk in the egg, egg yolk, and vanilla until glossy and slightly lighter in color.
In another bowl, whisk together the flour, cornstarch, baking soda, and salt. Stir the dry ingredients into the butter mixture with a spatula just until combined, then fold in 1 1/4 cups of the chocolate chips.
Press the dough evenly into the prepared pan, all the way to the edges. Scatter the remaining 1/4 cup chocolate chips over the top and press them in lightly.
Bake for 18 to 22 minutes, until the edges are golden brown and the center looks just set. Let the cookie cake cool completely in the pan, about 1 hour.
For the frosting, beat the butter, powdered sugar, almond milk, and vanilla on medium-high speed for 2 to 3 minutes, until light and fluffy.
Transfer the frosting to a piping bag fitted with a large star tip and pipe a border around the edge of the cookie cake. Top with the sprinkles and slice into wedges.