In the mixer bowl - beat the butter, margarine, brown sugar, and sugar together until fluffy.
Add the egg and vanilla - beat well.
Add in the flour, pudding powder mix, baking soda, and salt. Slowly stir first and then beat until all ingredients are completely incorporated.
Slowly stir in the chocolate chips.
Fold in ½ of the M&M's.
Remove the beater, cover, and refrigerate overnight. Don't skip this step.
The next day preheat the oven to 350 degrees.
Roll the dough into balls - it will be quite firm.
Place the dough balls on a baking sheet and bake the cookies for 9-12 minutes. (mine were perfect at 11)
Remove and immediately top each cookie with a few white chocolate chips and M&M's before they cool.
Leave the cookies on the baking sheet to cool.
Start with a cold cookie sheet with each batch to ensure that the cookies bake consistently.
Store the cookies in an airtight container at room temperature.