Preheat oven to 350 degrees F and grease a 9x13" baking pan.
Cream room temperature butter and both types of sugar together until the color has become very pale, the texture is soft and fluffy, and the volume has almost doubled in size.
Add in corn syrup and vanilla and mix to combine.
Add in the eggs and yolk and mix again until mixture is homogeneous.
In a separate bowl, whisk flour, baking powder, baking soda, salt, and corn starch.
Add dry ingredients to wet ingredients and mix until just combined.
Add in sprinkles, morsels, and any other mix-ins and mix until combined.
Press the dough into the 9x13" pan and flatten with your hand. Sprinkle more sprinkles and morsels on top for decoration.
Bake at 350 degrees for about 30-35 minutes. Don't let the top become too brown otherwise it'll lose its chewiness.
Cool completely and cut into squares or rectangles to serve.