Preheat oven to 450 F, with a rack in the center. Line a 12 muffin tin/pan with paper liners. If using paper muffin cups that are even with the cups in your pan,  spray the pan with non-stick baking spray so that the muffin tops do not stick.

Batter

Whisk the vegetable oil, Greek yogurt, sourdough discard, brown sugar, vanilla and egg in a large mixing bowl, until no trace of the egg or discard remain.

Sift the all-purpose flour, baking soda and cocoa powder into the wet mixture. Using a spatula, gently fold the dry ingredients with the wet ingredients until no trace of the  flour remains.

Pour the hot milk and hot espresso into the batter, mix well. The batter is thin at this point, that is ok.

Fold the chocolate chips into the batter.

Using a large cookie scoop or cup, divide the batter in the muffin tins. The batter should be almost to the top of the cupcake liner. Sprinkle the tops of the muffins with extra chocolate chips if you like.

Place the muffin tin(s) into the oven on the middle rack. Immediately turn the oven down to 375F. Bake the muffins for 20-25 minutes or until a toothpick comes out clean.

DO NOT open the oven door too soon or your muffins will sink and your work will be for naught.

Let the muffins cool in the tin for a few minutes before you turn them out onto the wire rack.

Enjoy the muffins warm or cooled.