Preheat the oven to 350 F. In a medium bowl, whisk together flour, pudding mix, salt, and baking soda. In a large bowl, mix together butter and sugar until fluffy. Add in egg and vanilla extract and mix until smooth.
Add in egg and vanilla extract and mix until smooth. Add in the flour mixture, and mix until it is fully combined. Fold in the M&Ms. Press the cookie dough evenly into a greased 8-inch square baking dish. Bake for 20-25 minutes.