In a medium bowl, sift the flour with the baking soda and salt.
In a large bowl, cream the butter with the two types of sugar with an electric mixer or hand whisk for 3 minutes until it turns light in color and fluffy.
Add the eggs one at a time, beating for 30 seconds after each addition. Beat in the vanilla extract.
Mix in the flour with a rubber spatula until smooth. Add the m&m's* chocolates and integrate with the same spatula until they are incorporated throughout the batter.
Portion the dough into 36 small balls the size of 2 tablespoons each, place on a plate, cover with plastic wrap and refrigerate for 2 hours or overnight.
Preheat the oven to 374º F (190° C). Place baking paper on a large baking tray. Place the cookies on the tray at least 2 inches (5 cm) apart from each other as they spread a lot during baking.
Bake the cookies for 8-10 minutes until they are firm and lightly browned on the edges but not quite in the center. Remove them from the oven and let them cool down completely on a wire rack.
The cookies can be stored in a well-sealed tupperware container for up to 1 week.