Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9-inch round cake pan or a 9x13 inch baking dish for a rectangular cake.
Step 2: In a large bowl, combine the melted salted butter, light brown sugar, and granulated sugar. Whisk until smooth and well combined.
Step 3: Stir in the vanilla extract and room temperature eggs one at a time, mixing well after each addition until fully incorporated.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, corn starch, baking soda, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix.
Step 6: Gently fold in the chocolate chips, M&Ms, and sprinkles until evenly distributed throughout the dough.
Step 7: Press the cookie dough evenly into the prepared cake pan. You can use your hands or a spatula.
Step 8: Bake for 25-30 minutes, or until the edges are golden brown and the center is set but still looks slightly soft. Remember, it will continue to cook as it cools.
Step 9: Remove from the oven and let the cookie cake cool completely in the pan on a wire rack before frosting.
Step 10: While the cookie cake cools, prepare the frosting. In a stand mixer with the paddle attachment (or a large bowl with an electric hand mixer), cream the room temperature salted butter until light and fluffy.
Step 11: Gradually add the powdered sugar, one cup at a time, mixing on low speed until combined, then increase speed to medium until fluffy.
Step 12: Add the vanilla extract and ½ tsp salt. With the mixer on low, slowly drizzle in the heavy cream, 1 tablespoon at a time, until your desired frosting consistency is reached. You may not need all 5 tablespoons. Beat on medium-high for 1-2 minutes until light and airy.
Step 13: Once the cookie cake is completely cool, spread the frosting evenly over the top or pipe a decorative border. Decorate with extra M&Ms or sprinkles if desired.
Step 14: Slice and serve! Enjoy your delicious Monster M&M Cookie Cake.