Set aside a small handful of M&M's for garnish, then roughly crush the rest with the back of a spoon or a rolling pin.
Add the ice cream, milk, crushed M&M's, and salt to a blender.
Blend on high for 15–20 seconds — pulse rather than blend continuously so you keep some texture instead of turning everything into liquid.
Taste and adjust — add more milk for a thinner shake or a few more crushed candies for extra flavor.
Pour into a chilled glass, top with whipped cream and whole M&M's, and serve immediately.