Step 1

In a stand mixer fitted with the paddle attachment, cream the butter, brown sugar and granulated sugar until light and fluffy, about 4 minutes.

Step 2

Add the eggs, one at a time, beating well after each addition. Add the oil, sourdough starter, milk and vanilla. Mix well. Scrape down the sides of the bowl as needed.

Step 3

In a separate bowl, whisk together the all purpose flour, bread flour, baking powder, baking soda, and salt. Add this to the wet mixture (in batches) and mix on low speed until just combined.

Step 4

Fold in the the chocolate chunks with a wooden spoon. Check the bottom of the bowl to make sure the ingredients are fully blended. The dough should be soft and not stick to your hands.

Step 5

Cover and chill the dough for 1 hr. The dough can be baked cold.

Step 6

Preheat your oven to 350 F. Set the oven rack to the top third, about 6 inches from the heat source. Line 2 rimmed baking sheets with parchment paper.

Step 7

Portion the dough into 2 heaping tbsp. balls (about 30 g each). Then gently roll by hand like a meatball. Place onto the baking sheets 3 across and 4 down, about 2 inches apart. Bake 1 pan at a time.

Step 8

Bake the cookies for 11-12 minutes. They will be light golden brown when ready. Cool on the baking sheet for 15 minutes to set up, and then transfer to a wire rack.