Heat oven to 350 degrees. In the bowl of an electric mixer fitted with paddle attachment, cream butter and sugar on medium speed until fluffy. With mixer running, add corn syrup. Turn speed to low; add flour. Blend until incorporated.
Line an 11-by-17-inch baking pan with a Silpat mat (a French nonstick baking mat). Pour 1 cup batter onto the Silpat; use an offset spatula to spread the batter evenly over mat. Bake until golden brown and lacy, about 12 minutes, rotating once for even baking. Transfer to a heat-proof surface to cool, 1 to 2 minutes.
Using a scalloped pastry wheel, cut the cookie into pieces about 1 1/2 inches longer than height of bag; place mold on cookie. Using an offset spatula, wrap cookie around mold, and press seams together. To make the bottom of bag, fold overhanging edges as you would wrap the bottom of a present. Let cookie harden on mold 1 1/2 minutes. Remove mold, and repeat with remaining cookie. If cookie becomes too hard to bend, return to oven 1 minute. Repeat the baking-and-folding process with remaining batter.
Store in an airtight container up to 1 week.
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