Preheat oven to 350°F (175°C). Grease a 9-inch round cake pan or springform pan.
In a large bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy.
Beat in the egg and vanilla extract until well combined.
In another bowl, whisk together the flour, baking soda, and salt.
Gradually mix the dry ingredients into the wet ingredients until just combined.
Fold in the chocolate chips.
Press the dough evenly into the prepared pan. Decorate with M&M’s on top if desired.
Bake for 20-25 minutes, or until golden and set in the center.
Let cool completely before slicing and serving.