In a large bowl, mix the starter, milk, and flour; cover and let rest at room temperature for 1.5 to 2 hours.
Preheat the oven to 350°F. Prepare a mini-muffin pan (this will make 4 dozen mini muffins but you could also bake this in a 9 x 13-inch baking pan) but spraying with cooking spray.
In the bowl of an electric mixer, beat together the sugar, oil, vanilla, salt, baking soda, cocoa and instant coffee.
Add the eggs one at a time, beating well after each addition.
In three additions, add the sourdough mixture into the wet chocolate mixture and keep mixing until fully incorporated; this will seem inconsistent at first, but the batter will smooth out as you continue to beat gently.
Add in the chips and continue to mix until incorporated well; spoon the batter (about one heaping tablespoonful) into the prepared pan.
Bake the bites for 20 minutes, until it springs back when lightly pressed in the center, and a cake tester inserted into the center comes out clean.