In the bowl of an electric mixer, combine the flour, water, sugar, starter, cocoa powder, olive oil, instant yeast, 2 teaspoons salt, and vanilla extract. 

Mix on medium speed until a smooth but sticky dough forms.

Cover and allow to rest for 30 minutes.

Do a set of stretch and folds. Grab the section of dough closest to you in the bowl and stretch it straight up above the bowl, and then fold it over to the opposite side. Rotate the bowl 90 degrees and repeat. Do this until you have done all four "sides" of the bowl. 

Recover and allow the dough to rest for 30 minutes. 

Do a second set of stretch and folds.

Recover the bowl and allow the dough to rest for 30 minutes. 

Do a third set of stretch and folds.

Recover the bowl and allow the dough to rest for 30 minutes.

Add 1 cup of chocolate chips on top of the dough and do a fourth set of stretch and folds.

Melt 2 Tablespoons of butter and spread in the bottom of a 9x13 baking dish.

Place the dough in the baking dish and gently spread it out as far as you can into the pan. Its ok if it doesn't fit perfectly now, it will reach better after it has finished proofing.

Cover and allow the dough to rest for another hour, until it has increased in size about 30-40% and is puffy. 

Preheat the oven to 425F and place a rack in the middle of the oven.

Melt an additional 2 Tablespoons of butter and pour over the dough. Sprinkle the remaining 1/2 cup of chocolate chips over the top of the dough evenly. 

Use your fingers to dimple the dough, pressing down through the dough, and working your way from one end of the pan to the other. 

Sprinkle with the flakey sea salt.

Bake for 25-30 minute.

Cool the bread for 10 minutes in the pan before removing to a wire rack and cooling the rest of the way.

Slice and enjoy.