Chop the butter into cubes and brown in a medium sized sauce pan on medium high heat.
Let chill for 30 minutes
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In the bowl of a mixer, add the sugar and butter, then cream until light and fluffy, about 2-3 minutes.
Add the egg yolks and mix for an additional 3 minutes, scraping down the sides of the bowl half way through.
Add the starter and vanilla, then mix until fully combined. It should be smooth and look like a cake batter.
Add the flour in three additions, scraping down the sides after each one.
Add the chopped chocolate and mix until just combined.
Cover the bowl in plastic wrap and let rest in the fridge for 24 hours.