Preheat oven to 350 degrees.
Place butter in a medium saucepan & cook over medium heat, stirring constantly, until melted and browned
(butter will foam & bubble as it cooks)
Once the butter is golden brown remove from the heat
Transfer browned butter to mixer bowl & let cool for several minutes
Add the brown sugar, granulated sugar, & espresso powder to the butter and beat until well combined
Beat in vanilla & eggs until just combined
Add flour, baking soda, and salt and mix until just combined
Stir in both varieties of chocolate chips slowly
Stir in 1/2 the M&M's - reserving the remaining candies for topping the cookies after baking
Portion tablespoon sized balls of dough and place on a large baking sheet
Bake for 11-12 minutes
Cookies will still be slightly underdone when you remove them from the oven. Do not overbake. Let sit on baking sheet for 5 minutes before removing to a cooling rack or eating.