In a microwave-safe bowl, melt butter and peanut butter; stir until smooth. Combine sugar and flour; gradually add to butter mixture and mix well. Beat in eggs and vanilla.
Spread into a greased 13x9-in. baking pan. Bake at 350° until lightly browned and edges start to pull away from sides of pan, 28-32 minutes. Cool on a wire rack.
Test Kitchen Tips
Whole wheat pastry flour is ¬made from soft wheat, which contains less gluten than hard wheat, the ingredient in regular whole wheat flour. Pastry flour produces lighter, more tender and flaky biscuits, pie crusts and baked goods. If you need a sub, try a 50-50 mix of cake or all-purpose and whole wheat flours.
Can't resist a taste before the party begins? Cut the bars and arrange them on a plate. Your guests will never guess there's a piece missing.