1. Preheat oven to 325 degrees Fahrenheit. Grease a nine- by 13-inch baking pan.
2. In a mixer bowl, combine margarine and sugar until grainy. Add egg and vanilla extract.
3. In a separate bowl, mix flour and baking powder. Scrape down sides of bowl and add flour mixture. Blend at low speed until thoroughly combined. (Dough will be firm.)
4. Wrap dough in plastic wrap and refrigerate for one hour.
5. Divide the dough in two. On a floured board, with a floured rolling pin, roll half of the dough to a nine- by 13-inch rectangle (to fit into a nine- by 13-inch pan), approximately 1/4-inch thick. Carefully place the dough in the bottom of the pan and part way up the sides. Refrigerate for another 10 minutes.
6. Spread half of jelly on the dough, layer peanut butter over jelly, and top with remaining jelly. Sprinkle jelly with confectioners’ sugar.
7. Roll second dough to nine- by 13-inch rectangle. Place on top and pinch down side edges all around inside of pan.
8. Bake at 325 degrees Fahrenheit for 35 to 40 minutes or until golden brown and firm to touch (even in center). Cool and cut into squares.