In a large mixing bowl, cream the butter and sugars. Stir in the vanilla, then the salt and flour until just combined. Add the M&M's® and stir. Turn the dough out onto a large sheet of plastic wrap and press into a slab around one inch thick. Wrap up in the plastic wrap and place in the fridge until cold and firm, then cut into tiny cubes.

In a large mixing bowl with a whisk attachment, whip the heavy cream until thick and stiff, around 3-5 minutes, depending on the mixer. Gently fold in the sweetened condensed milk and vanilla until thoroughly combined. Mix in the cubes of cookie dough, then pour into a 9x5 loaf pan and freeze overnight.