Preheat oven to 350 degrees. Prepare mini muffin tins by greasing well; set aside.
Beat butter, sugars, eggs & vanilla in large bowl until light and fluffy.
Combine dry ingredients in a medium bowl; blend into butter mixture until smooth.
Stir in Candy Corn M&M's.
Fill mini muffin tins with 1 Tbsp. dough each.
Bake at 350 degrees for 12-14 minutes.
Cool; remove from tins.
For frosting:
Blend butter, powdered sugar, and white chocolate at medium speed until combined.
Add milk and vanilla and blend at medium-high speed until mixture is smooth.*
Apply generously to cookie cups. Top with M&M's.
*The melted white chocolate will add some texture to your frosting. If you would prefer a very smooth texture, feel free to omit the white chocolate from the recipe.