Preheat oven to 350°F. Grease and line with parchment a 9 x9-inch baking pan
In a medium saucepot, melt butter and chocolate together over very low heat. Remove pot from heat and stir in sugar and vanilla until blended
In a small mixing bowl, whisk eggs until frothy then stir eggs completely into chocolate mixture.
Sift flour into batter and stir just until smooth. Pour batter into prepared pan, smoothing to the edges.
In mixing bowl, cream together peanut butter, butter and sugar. Add egg and vanilla and beat just until blended. In separate bowl, sift or whisk flour and baking powder together then stir into batter just until combined.
Carefully spread the peanut butter batter over the top of the chocolate batter. Use a knife to swirl the batters together, first horizontally and then diagonally.
Bake 20 minutes, or until toothpick inserted in center comes out almost clean. Cool completely on wire rack before cutting.
Do not cream brownie batter as for a cake because brownies are best when the batter has not had a lot of air incorporated into it.
The more the batters are swirled together, the more the brownies will be “marbled” after baking.
