Heat margarine, chocolate, milk and apricot preserves in small saucepan, whisking frequently, until chocolate is almost melted.
Remove from heat; whisk until chocolate is fully melted. Whisk in egg yolk and vanilla; mix in combined flour, Equal®, baking powder, and salt until smooth.
In a separate bowl, beat egg whites and cream of tartar to stiff peaks. Fold chocolate mixture into egg whites. Fold in walnuts, if desired. Spray an 8 inch square baking pan with nonstick cooking spray and pour in batter.
Bake in preheated 350°F oven until brownies are firm to touch and toothpick comes out clean, 18 to 20 minutes (do not overbake). Cool on wire rack. Serve warm or at room temperature.