Line a 9 inch pan with parchment paper, covering the bottom and the sides.
Soften the butter in the microwave for 10 to 15 seconds. In a medium bowl, use an electric mixer to combine the butter, white sugar and brown sugar until combined. Mix in the peanut butter, scraping down the sides of the bowl to incorporate. Add the flour and vanilla, and mix until the flour starts to incorporate. Add the oats, 1 cup at a time. Stir in the M&M's.
Gently push the dough evenly into the bottom of the pan.
TOPPING
In a medium, microwave-safe bowl, combine the chocolate chips and whipping cream. Microwave in 30 second increments until the chocolate starts to melt, stirring occasionally. Once the chocolate is melted, stir vigorously until the chocolate is smooth and free of lumps.
Pour the ganache over the top of the cookie dough base, spreading evenly, and sprinkle with the M&M's. Allow the ganache to set at room temperature, and then refrigerate the bars until the cookie dough and ganache harden slightly, at least 1 hour. Lift the parchment paper by the edges to remove the bars from the pan before cutting.