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1. Coat an 8-inch square pan with cooking spray; line bottom with parchment paper. Coat the parchment with cooking spray, and set pan aside.
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2. Put sugar, corn syrup, and 3/4 cup water in a small saucepan. Cook over medium heat, stirring, until sugar is dissolved. Stop stirring; let mixture come to a boil. Raise heat to medium-high; cook until mixture registers 260 degrees on a candy thermometer.
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3. Meanwhile, sprinkle gelatin over 3/4 cup water in a heatproof bowl; let stand 5 minutes to soften. Set the bowl with the gelatin mixture over a pan of simmering water; whisk constantly until gelatin is dissolved. Remove from heat, and stir in extract if you choose to use it; set aside.
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4. Beat egg whites in the bowl of an electric mixer fitted with the whisk attachment until stiff (but not dry) peaks form.
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5. Whisk gelatin mixture into sugar mixture; with mixer running, then gradually add it into egg whites. Mix on high speed until very thick, 12 to 15 minutes.
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6. Pour mixture into lined pan. Working quickly, drop dots of red food coloring across surface of marshmallow. Using a toothpick, swirl food coloring into marshmallow to create a marbling effect. Let marshmallow stand, uncovered, at room temperature until firm, at least 3 hours or overnight.
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7. Cut into squares.
