Combine the flour, salt & baking powder into a bowl, mix and set aside.
In a mixer bowl, cream together the softened butter & sugar.
Once smooth, add the eggs & vanilla and beat well.
Slowly stir in the dry ingredient mixture until fully incorporated, but don't over-mix.
Fold in the M&M’s.
Form the dough into a log and then wrap it tightly with plastic wrap. Refrigerate for at least 3 hours but overnight is best.
Once chilled, preheat the oven to 350 degrees.
Slice the dough into rounds to your desired thickness. The thinner the blade used to slice them, the better. A thick blade will tend to squish the dough.
Place the rounds on a parchment-lined baking sheet.
I usually add a few more whole M&M”s to the tops of the cookies at this time.
Bake in the center of a preheated oven at 350 degrees for approximately 8-10 minutes.