Cube the butter and put it into a bowl. Place the bowl of butter in the fridge until all your ingredients are measured.
In a large mixing bowl, whisk together the flour, corn starch, salt and baking soda. Make sure all of the ingredients are fully combined. Set the dry ingredients aside.
In the bowl of a mixer, add the light brown sugar and the granulated white sugar. Mix on low with the paddle attachment until the sugars are combined.
Add the cold butter to the sugar and mix on low with the paddle attachment until the butter is folded into a sugar, and a cold, buttery sugary paste is formed. If using a hand mixer, run it through the butter and sugar on the lowest speed. You can also use your hands to fold the butter into the sugar.
Add the egg yolks and egg, and mix with the paddle attachment on medium speed, until combined and bright yellow. If using a hand mixer, you can continue to mix on low, or medium low, and guide the eggs into the sugary mixture by moving the mixer around the bowl.
Once the eggs are combined, add the flour mixture and slowly fold it into the sugar mixture until a dough is formed. It should be firm, dense and slightly sticky.
Add the funfetti sprinkles and fold in slowly until combined to your liking.
Wrap the dough in cling wrap and put in the freezer for at least 1 hour to be safe. You can leave the dough in longer until you're ready to bake as well. Your dough must feel FIRM before you cut it. The time of freezing depends on a variety of factors, such as the temperature of your freezer, the temperature of the dough before you put it in (which can be caused by the temperature of your kitchen) and the temperature of your butter. Just make sure the butter is firm before cutting. This step isn't 100% *necessary* if your dough feels firm, but it does help prevent the cookies from going flat and I highly recommend.
Preheat the oven to 410 degrees and prepare a sheet pan.
Remove the dough from the freezer and cut into equal-ish size squares. There should be about 8 total. Each square should be firm and chilled, and kind of tough to roll into a ball. If the dough molds super easily and still feels really sticky, the dough still needs to sit in the freezer longer. Roll each square into a large ball, big enough to rest in your hand. Place the balls onto the sheet pan and place the pan into the oven in the middle rack. Bake for 12-15 minutes, until the outside is golden brown and a little crackled.
Let the cookies sit for at least 10 minutes before eating. I recommend eating them while they're warm and gooey because that's when they taste the most like cake batter. After they set, they begin to take on more of a sugar cookie vibe since the egg yolks cook and set inside the cookie. Either way, I think you'll love these! If you want to make them and serve them another day, they keep for a week in the fridge. You can also microwave before serving, since the microwave heats the inside out. ENJOY!