Heat oven to 325.

In a mixing bowl, add cake mix, butter, peanut butter and eggs. Mix with beater for about a minute, combining all ingredients well. Set aside 1-½ cups batter. Evenly press remaining batter into a 9x13 pan using your fingers.

On the stove, melt chocolate chips, sweetened condensed milk and vanilla. Use medium heat and stir constantly so the chocolate doesn't burn. When completely melted, remove from  heat and pour into 9x13 pan. Spread chocolate evenly.

Using reserved batter, place cake mixture on top of chocolate. I used my fingers to grab a small chunk of batter and place on top of the chocolate.

Bake 20-25 minutes. Cool completely.

To cut into bars: Use a metal spatula and loosen the bar edges all the way around the edge of the pan. Cut into about 35 bars (5 rows x 7 rowand serve. I found a sharp knife to be the best way to cut, and dragging it seemed to work well. Store covered if not serving right away.