Preheat oven to 350°. Grease a 13 x 9 x 2-inch baking pan.
In a small mixer bowl at medium speed, beat cream cheese until smooth. Add 1/3 cup sugar, 1 egg, 2 tablespoons flour, 3/4 teaspoon vanilla, and peppermint extract if desired and beat until blended. At high speed, beat until smooth set aside.
In a 3-quart saucepan over low heat, melt butter and chocolate. Remove from heat. Stir in 2 cups sugar. Stir in 4 eggs, one at a time, until blended. Stir in 1 1/4 cups flour, 1 teaspoon vanilla, and salt until smooth. Spread in baking pan. Spoon cream cheese mixture over top and swirl with a spatula to make a marble pattern.
Bake 35 to 40 minutes until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Store, covered, at room temperature.
High Altitude Adjustments: At 6,000 feet, no adjustments needed.
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