1. Pour sweetened condensed milk into the top of a double boiler; place over boiling water. Reduce heat to low; simmer for 1 1/2 hours or until mixture is thick and caramel-colored, stirring occasionally. Cool slightly.
2. Preheat oven to 350°.
3. Place chocolate in a microwave-safe dish, and microwave at HIGH 30 seconds or until almost melted, stirring after 15 seconds. Combine melted chocolate, 1/2 cup sugar, and butter in a medium bowl; beat with a mixer at high speed until well blended. Add vanilla and egg whites; beat until well blended. Weigh or lightly spoon flour into a dry measuring cup. Combine flour, 1/3 cup cocoa, baking powder, pepper, and salt. Add flour mixture to sugar mixture; beat just until blended. Stir half of sweetened condensed milk into batter. Spoon batter into a 9-inch square metal baking pan coated with cooking spray. Dollop the remaining sweetened condensed milk by the spoonful over chocolate mixture; swirl together using a knife. Bake at 350° for 19 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Cut into 16 squares.