Grease medium-size rectangular baking dish, or spray with vegetable-based cooking spray.

Line the bottom of the dish with half of the graham crackers. Set the other half aside.

In a medium microwave-safe bowl, combine graham cracker crumbs, granulated sugar, brown sugar, butter and half-and-half (or heavy cream). Microwave on high heat for 30 seconds, uncovered. Remove bowl from microwave and stir. Return to microwave and heat for another 30 seconds. Repeat this step until mixture is melted and sugars have dissolved, or about 2 minutes longer - stopping to stir.

Pour half of the mixture over the graham crackers in the prepared pan. Top with the remaining graham crackers set aside earlier. Pour remaining mixture on top of the graham crackers, creating a set of layers. Crackers, butter mixture, crackers, butter mixture.

In a small microwave-safe bowl, combine chocolate chips and peanut butter. Microwave on high for 30 seconds, uncovered. Remove from microwave and stir. Return to microwave and heat for another 30 seconds. Stir. Repeat this process until chocolate and peanut butter has melted and is smooth - or for another 1 1/2 minutes.

Pour the chocolate mixture on top of the layered graham cracker bars. Use a spoon or spatula to smooth the chocolate evenly over the graham crackers. Refrigerate at least 2 hours to set up.

Cut into bars and store in a cool place.