Preheat oven to 350°F. Line a muffin tray with muffin liners or grease so that batter will not stick. Set aside.
Mix flaxseed and water in a small bowl. Set aside to thicken.
Combine soy milk and orange juice and set aside. The milk will curdle as it sits.
In a medium bowl, sift together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
Mix tahini and melted chocolate in a large bowl. If your tahini is too thick, microwave it briefly till it becomes runny. Add orange zest and vanilla extract.
Add the half the dry ingredients into the wet ingredients, briefly mix, then add the rest of the dry ingredients. Mix until just combined, then add chocolate chips and mix until chocolate chips are evenly distributed. Be careful not to over-mix.
Add spoonfuls of batter to the muffin liners, filling them about 3/4 of the way. Top with orange segments if using, then bake for 22 minutes. Check for doneness by inserting a toothpick in the center of a muffin -- it should come out almost clean. Let cool completely before serving.