Preheat your oven to 425°F and line a muffin tin with paper liners.
Mix dry ingredients: In a large bowl, combine the flour, cocoa powder, baking powder, baking soda, and salt.
Mix wet ingredients: In another bowl, whisk together the sourdough discard, oil, sugar, eggs, yogurt (or sour cream), milk, and vanilla until smooth.
Combine: Gently stir the wet ingredients into the dry ingredients until just combined. Avoid overmixing to keep your muffins tender. Fold in the chocolate chips.
Fill muffin tins: Divide the batter evenly among the muffin cups.
Bake: Start at 425°F for 5 minutes, then reduce the oven to 350°F and bake for an additional 15–18 minutes. Muffins are done when a toothpick inserted in the center comes out clean.
Cool and serve: Let the muffins cool slightly before serving.