Preheat oven to 180 C (350F). Line baking sheets with parchment paper.
In a small bowl, whisk together flour, baking soda, salt and nutmeg.
In a large bowl, combine the brown sugar and white sugar. Using a wooden spoon, press the butter into the sugar until mixture is smooth and combined.
Add the egg and vanilla extract and stir well, then slowly stir in the flour mixture.
Stir in the chocolate chips, and either bake immediately, or allow to chill 30 minutes in the fridge if you have time.
Place 12 slightly rounded tablespoons of cookie dough onto each prepared baking sheet. About 1/2 inch apart. Press lightly on each dough circle to flatten the dough slightly. If using M&M's, press a few on top of each dough circle.
Bake in the preheated oven for 6-8 minutes or until just golden brown on top. Remove from oven and let cool another 2 minutes on the baking sheet before removing to a wire rack to cool
You can either bake any extra dough you have in the same manner, or refrigerate dough tightly covered for 3 days, or form cookie dough balls and freeze for up to 3 months.