To make the cookies, place the butter and chopped dark chocolate in a small bowl. Melt it in the microwave for 30 second intervals, stirring in between until the chocolate has melted completely. Then allow it to cool down.
Mix the flour, baking soda, cornstarch, salt and cocoa powder in another bowl.
In a separate bowl, whisk the eggs, egg yolk, sugars and vanilla. Then slowly whisk in the cooled melted chocolate.
Then add the flour mixture to the chocolate mixture in batches. Mix until everything is combined.
Then allow the dough to chill in the fridge for 2 hours of overnight..
In the meanwhile, start preparing the rhubarb. Slice the stalks in two lengthwise. Then finely slice them.
Place the butter with the sugar in a pan over medium heat. Once the butter has melted, add the rhubarb and vanilla and cook for 1 minute while stirring. Then transfer the rhubarb to a fine mesh sieve with a bowl underneath to catch all the liquid. Let it sit there until the chilling time for the cookie dough is up.
Once the cookie dough has chilled, preheat your oven to 180C/350F and line a baking sheet with parchment paper. Discard the juice from the rhubarb and place the rhubarb in a small bowl.
Take the cookie dough out of the fridge and grab about 2 tablespoons of cookie dough and roll it into a ball using your hands. Place it on the prepared baking sheet. Repeat for the rest, leaving some space in between each ball. Place about 1 teaspoon of the rhubarb on top of each ball and press with your fingers so it sticks. Bake the cookies for 12 minutes. While a batch is baking, keep the remaining cookie dough in the fridge.
Let the cookies rest in the baking sheet for 10 minutes before transferring them to a cooling rack