Prep and line. Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.

Whisk the wet ingredients. In a large bowl, whisk the melted butter, granulated sugar, and brown sugar until smooth and glossy, about 30 seconds. Whisk in the egg and vanilla until fully combined. Stir in the sourdough starter until the mixture looks creamy and uniform.

Mix the dry ingredients. In a separate bowl, whisk the flour, baking soda, baking powder, and salt.

Combine. Add the dry ingredients to the wet ingredients. Stir with a spatula until only a few floury streaks remain. Fold in the chocolate chips and chopped chocolate until evenly distributed. Do not overmix.

Chill briefly. Cover and refrigerate the dough for at least 30 minutes and up to 24 hours. Longer chill equals thicker, chewier cookies.

Scoop. Use a 2-tablespoon scoop (about 35–40 g) to portion dough onto the prepared sheets, leaving about 2 inches between scoops. If you like picture-perfect puddles, press a few extra chips on top.

Bake. Bake 10–12 minutes, rotating the pans halfway. The edges should be set and lightly golden, but the centers will look soft and slightly underdone.

Pan-bang for crinkles (optional). For rippled edges, lift the pan a couple of inches and let it drop once or twice at the 8-minute mark, then finish baking.

Finish and cool. Sprinkle with flaky salt right out of the oven if using. Let cookies cool on the sheet for 5–7 minutes before moving to a rack. They’ll set up as they cool.