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In a bowl with an electric mixer, beat the egg whites and cream of tartar until soft peaks form.
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Meanwhile, in a small saucepan set over medium-high heat, combine the sugar, corn syrup, water, and salt. Bring the mixture to a boil and continue to cook until a candy thermometer registers exactly 240°F. Immediately remove from heat. Turn the mixture on low speed and carefully and gradually drizzle the syrup into the egg whites. Increase speed to medium-high and beat until the mixture is thick and glossy and looks like marshmallow fluff, about 7 minutes more. Add in the vanilla and beat until combined.
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Spread the marshmallow topping evenly over the chocolate fudge layer. Use a kitchen torch to gently torch the marshmallow layer until lightly golden. OR you can alternatively, place the pan under the broiler and broil for 1 minute, or until lightly golden and the top is toasted.
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Refrigerate the pan for at least 1 hour or overnight before cutting into squares and serving. The squares can be stored in an airtight container in the refrigerator for up to 5 days, or stored in the freezer (wrapped up tight) for a month.