Using a brownie or loaf pan, spray with non stick spray and layer with a sheet of parchment paper. Use bulldog clips for the edges to keep the paper in place.
Start with the chocolate layer: in a microwave safe bowl combine the chocolate chips, condensed milk and 2 tbsp of butter. Melt in the microwave for 30 seconds and then stir. Repeat until completely melted. Working quickly, pour into your prepared dish and spread flat ensuring you get into the corners. Place in the fridge to chill while you prepare the second layer.
For the peanut layer, combine the peanut butter, 1 cup of butter and vanilla in a microwave safe bowl. Heat for 30 seconds and then stir until smooth. Gradually mix in the icing sugar using a whisk or electric mixer until the icing is pale and fluffy. Pour the peanut butter mixture over the chocolate layer and spread smooth. Chill until firm, for 1-2 hours.