In mixer bowl - beat butter, margarine, sugar & brown sugar together until fluffy
Add egg & vanilla - beat well
Add in flour, pudding mix, baking soda & salt - slowly stir first & then beat until all ingredients are completely incorporated.
Slowly stir in chocolate chips
Fold in 1/2 the M&M's
Remove beater, cover & refrigerate overnight
The next day preheat oven to 350 degrees
Roll dough into balls - it will be firm
Place dough balls on baking sheet & bake cookies for 9-12 minutes (mine were perfect at 11)
Remove & immediately top each cookie with a few white chocolate chips & M&M's before they cool.
Leave cookies on baking sheet to cool
Start with a cold cookie sheet with each batch to ensure that the cookies bake consistently.
Store in an airtight container at room temperature