First, in a large bowl or the bowl of a stand mixer, measure out and mix together the sourdough starter, water, milk, and brown sugar. Mix until a slurry is formed.
Next, add the flour, ground cardamom, butter, and egg. Mix this by hand, Danish dough hook, or the dough hook attachment until the dough is just coming together and is shaggy.
Let rest (autolyse) for 30 minutes covered with a tea towel. Once the 30-minute rest is complete take and knead either by hand or with the dough hook attachment for 5 minutes. Make sure to scrap the sides of the mixing bowl. The dough will be a little sticky…don’t add any extra flour.
After the kneading, let the dough rest, in the bowl (Transfer to a different bowl if you used a stainless-steel bowl to mix.), on the counter covered with plastic wrap for 1 hour. Once the hour is up transfer to the fridge for the bulk fermentation for 12-18 hours.
The next day once the bulk ferment is complete you will take your dough out of the fridge and transfer it to a clean work surface. You can do this on a lightly floured surface if needed but I usually do not use any flour.
Taking a rolling pin, flatten the dough into a rectangle about 18” x 22". Making sure to lift up the dough periodically to keep it from sticking to the counter. This is a slightly sticky dough but still easy enough to manage.
Once it is the correct size take the filling ingredients and in a small bowl mix them together until well incorporated.
Spread this mixture onto the dough covering everything but the very edges. Next, take the long side and fold in the dough to a third of the way. Repeat on the other side. See picture.