Add 225 g unsalted butter to a light-coloured pan or skillet. Melt over medium low heat. Once melted, add 30 g powdered milk while whisking constantly. Keep heating while stirring regularly, until the milk solids in the butter turn brown and the butter smells nutty and very buttery.
Transfer the butter to a large mixing bowl. While it's still warm, add 220 g granulated sugar and 220 g brown sugar, and stir until combined.
Add 3 large egg yolks, 1½ tsp vanilla extract, 240 g sourdough discard or active starter, and 2 tbsp maple syrup and stir until smooth and completely combined.
Add 450 g all-purpose flour, 1½ tsp baking powder, ¾ tsp baking soda, and 1½ tsp fine sea salt. Fold until almost combined.
Add 500 g chocolate chips or chunks and mix until distributed evenly through the dough. You may find it easier to use your hands here.
Use a cookie scoop or a spoon to portion out the dough into balls. Place on a lined cookie sheet and cover. Refrigerate for at least 2 hours and up to 5 days.
Preheat the oven to 350˚F (180˚C). If baking two sheets at once, arrange your oven racks so one is in the top third and the other is in the bottom third.
Arrange dough balls on lined baking sheets so they're at least 2" apart. you may want to form the dough balls into towers to prevent them from spreading too much in the oven. If you wish, sprinkle on some flaky sea salt on top of each cookie before baking.
Bake for 15-18 minutes, or until cookies have spread and are starting to look golden brown on the edges. Leave cookies to cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.
You can leave these on a tray or plate at room temperature for up to 3 days.
