Cake:
Preheat the oven to 350 degrees F. Line 24 cupcake tins with liners (or spray with nonstick cooking spray).
Combine the dry ingredients in a large bowl and whisk together.
Beat in the oil, milk, vanilla and eggs and use a hand mixer set on medium to mix for 2 minutes.
Slowly add the boiling water and whisk to incorporate that into the cake mix.
Fill the cupcake tins 2/3 full with the mixture, press 1 peanut butter M&M into the batter (*optional) and bake for 22 minutes or until a toothpick comes out clean.
Allow to cool in the tins for 10 minutes then place on a wire rack to continue cooling.
Frosting:
Whip the peanut butter for 1 minute in the bowl of a stand mixer fitted with the paddle attachment. Add the butter and continue to whip.
Slowly add the sugar,salt and vanilla and increase the speed to medium. Mix for 2 minutes.
Reduce the speed and carefully add the milk, 1 tablespoon at a time, to reach the desired consistency.