Melt the butter in a small ramekin or mug and allow it to cool slightly. Stir in the egg yolk, almond flour and sugar-free sweetener and mix well until thick dough has formed. Stir in the chocolate chips.
Microwave on high for 40 to 60 seconds, or until the cookie has set up around the sides but is still slightly gooey in the middle (mine takes about 45 seconds).