Preheat the oven to 350℉. Line a 9x13 inch baking pan with foil and spray it with nonstick spray.
Make the cookie crust: In a large bowl, use an electric hand mixer to combine the butter and sugars until light and fluffy. Add the peanut butter and beat until creamy. Add the eggs and vanilla and beat again.
In a separate bowl, whisk together the flour, oats, salt and baking soda.
Slowly beat the dry ingredients into the wet ingredients. Add the milk and beat again. Stir in the chocolate chips and 3/4 cup M&M's.
Press half of the cookie dough into the bottom of the prepared pan. Set the rest aside.
Make the cheesecake layer: In a large bowl, use an electric mixer to beat the cream cheese until creamy. Beat in the sugar until smooth. Add the melted chocolate and beat until incorporated. Beat in the eggs.
Spread the cheesecake layer over the cookie dough layer in the pan. Crumble the remaining cookie dough over the cheesecake layer. Sprinkle remaining M&M's on top.
Bake for 35 minutes. Remove the pan from the oven and place on a wire rack. Let the cheesecake cool for 1 hour, then refrigerate for 4 hours or until completely chilled.
Cut into 24 bars before serving. Keep bars refrigerated until ready to serve.