Heat oven to 300 degrees. Grease a 13x9 inch pan and set aside.
In a large bowl, whisk together the flour, dry cake mix, and baking soda. Set aside.
In the bowl of a stand or handheld mixer, add the butter and sugars, then beat with a paddle attachment on medium until smooth and slightly fluffy, around 3 minutes. Add the egg and beat on high for 1 minute, then add the vanilla extract and beat on high until combined. Add the flour mixture and beat on low until just combined. Mix in the white chocolate chips and sprinkles on low until evenly distributed.
Scoop batter into 13x9 inch pan and press down by hand until evenly distributed*. Bake for 35-40 minutes, or until golden brown on top and a toothpick inserted into the center comes out mostly clean. Place on a cooling rack and allow to cool completely before frosting.
For the Buttercream Frosting
In a large mixing bowl, beat together the butter, light corn syrup, vanilla, and kosher salt until light and fluffy, around 5 minutes. Slowly beat in the powdered sugar in batches, adding the heavy cream as necessary to thin the buttercream. Once the desired consistency is reached, spread on cookie bars. Reserve any remaining frosting in an airtight container in the fridge for up to 2 weeks.
*Because I hate high lips around the edge of the pan, I usually mound the dough higher in the middle and leave a thin edge around the pan. That way, as it bakes and spreads, it evens out better in the pan.