1Preheat oven to 350 degrees F.Grease and flour an 8-inch square pan.
2Cream butter and 1/2 cup splenda sweetener until fluffy.Add sodium-free salt and vanilla.Separate eggs
and reverse the whites.Add yolks to the sugar mixture;beat until creamy.
3Combine 1 1/2 cups flour and baking powder;add alternately with milk to egg yolk mixture.Add blueberries and peanut butter to the batter.
4In a separate bowl,beat white until soft peaks form.Add 1/4 cup brown sugar,1 tablespoon at a time,and beat until stiff peaks form.Fold egg white into batter.Pour into prepared pan.Bake for 50 minutes,or until a toothpick inserted into the center of the cake comes out clean.
5For the frosting,in a medium mixing bowl,combine whipped topping,cream cheese,peanut butter and cinnamon,whisk together until blended,Spread the frosting onto cake.