Preheat the oven at 425°F (220°C), rack positioned in the middle.
In a large bowl, add cocoa powder. You can sift it if it’s too lumpy. Bloom the cocoa powder by adding hot milk over it, whisking until the ingredients are fully combined.
Whisk the remaining wet ingredients into the batter: espresso, Greek yogurt, oil, brown sugar, vanilla extract, egg, and sourdough discard, whisking them together until combined.
In a medium bowl, mix flour, salt, and baking soda. Add this to the batter, whisking until all the flour is absorbed and no dry lumps remain.
Add ¾ of the chocolate chips, saving some for topping the muffins.
With the help of a large spoon or ice cream scoop/cookie dough scoop, fill the muffin cups until they are about ¾ full with the batter, then top them with the remaining chocolate chips.
Bake for 5 minutes at 425°F (220°C), then lower the temperature to 375°F (190°C) and bake for about 15 minutes more, or until you insert a toothpick in the middle and it comes out with crumbs only, not runny batter.
When baked, remove the muffins from the oven and wait for about 5 minutes before transferring them to a cooling rack. Wait another 10 minutes before serving.