Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9x13 inch pan.
In a large bowl, cream together butter and sugar until light and fluffy. Beat in eggs, one at a time, then stir in vanilla.
In a separate bowl, whisk together flour, baking powder, and salt. Gradually beat into creamed mixture alternately with milk. Divide batter in half.
Melt chocolate chips and peanut butter together in the microwave or a double boiler. Stir until smooth. Add to one half of the batter.
Pour both batters into the prepared pan. Drag a knife through the batters to marbleize. Bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean.
Cool before serving.
Interesting Facts
This cake is an easy and delicious way to make a treat for the whole family.
The marble effect comes from the swirling of the two batters together.
The combination of chocolate and peanut butter is a classic flavor combination that never fails to please.