First you'll melt the butter, then add the peanut butter and melt. Then just whisk until it's nice and creamy.
Next, whisk in the brown sugar and add eggs and vanilla. Then whisk in the flour.
Pour batter into prepared dish and bake for about 25 minutes, until edges are set but there is still a little gooeyness on toothpick tester in the center. Do not over bake.