Preheat oven to 350 degrees F. Line an 8-inch pan or a quarter sheet pan (6 ½ by 9) with parchment paper.
Mix together the flour, soda and salt; set aside.
Beat the butter and both sugars in a mixing bowl. When light, beat in the vanilla. Add the egg and beat until it’s mixed, then gradually stir in the flour mixture, followed by the pretzel pieces, chocolate and M&Ms.
Spread mixture evenly in pan and bake on center rack for about 25 minutes or until golden brown. Let cool completely. Lift from pan and cut into bars.