Position an oven rack in the center of the oven and preheat to 325 degrees F. Butter a 12-by-9-by-1/2-inch jelly roll pan and line it with parchment paper, then butter the paper.
Whisk the flour, cornstarch and salt in a bowl. Beat the butter and sugar in an electric stand mixer fitted with the paddle attachment on medium speed until light and fluffy, 5 to 7 minutes. Slowly add the flour mixture and beat on low until the dough is just combined; do not overwork. Stir in the sprinkles to incorporate.
Transfer the dough to the prepared pan, spreading it out as evenly as possible with a spatula. Cover with plastic wrap, then use your hands to press and flatten the dough into a smooth even layer. Remove the plastic and cut the dough with a crinkle cutter into approximately 3-by-1/2-inch fries, leaving the fries in place in the pan.
Bake, rotating the pan from front to back halfway through, until golden, 50 to 60 minutes. Let cool for 5 minutes in the pan, then re-cut the bars with the crinkle cutter along the previous cuts. Carefully lift the fries off the parchment with a small spatula and transfer to a wire rack to cool completely. Serve with warmed chocolate sauce or strawberry sauce for dipping.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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