Soften the butter in the microwave (don't have to melt). Roughly chop the chocolate bar into 5 mm - 1 cm cubes.
Add sugar and vanilla extract to the soft butter and use a whisk to mix. It'll take time, but mix until it's fluffy and white.
Once it's become fluffy and white, add the eggs a little at a time. Add, mix, add more and mix again.
After mixing, add the flour. Use a wooden or rubber spatula to mix well. It may look a bit crumbly, but it'll come together.
Once you've mixed it together, add the chocolate (the photo shows two chocolate bars worth). Mix everything together.
Use a large spoon to scoop out onto a baking pan. Don't make them too nice, just drop them down in chunks. That's cuter. See Helpful Hints.
Bake in a preheated oven at 190℃ - 200℃ for 10-12 minutes. Adjust accordingly and bake, until they have a light golden brown color.
Transfer on a rack, or sieve to cool and they're complete.
Story Behind this Recipe
I wondered if it'll be delicious to make cookies out of shortbread dough. I did and it was crispy and delicious. The amounts are just an estimate, but if you observe the ratio of sugar : butter : flour as 1 : 2 : 3, and then just add an egg then you can make these whenever you want. I've been making these ever since I was in high school.
As household ovens will vary, adjust the baking temperature and time accordingly.
Once you've put the dough on the pan, for the finishing touch, use a spoon to make the top a bit jagged for a rough look.