Preheat oven to 425°F. Spray a 12-count muffin pan with nonstick spray or line with cupcake liners. Set aside.
Place the chopped chocolate and oil in a large heat-proof bowl. Microwave on high in 10 second increments, whisking after each until completely smooth. Whisk in the sugar, yogurt, eggs and vanilla extract until combined and set aside.
In a medium bowl, whisk together the flour, cocoa powder, espresso powder, baking soda and salt. Add the dry ingredients to the wet ingredients and stir until just combined. Stir in the chocolate chips.
Evenly fill each liner with batter and top chocolate chips. Bake for 5 minutes and then reduce heat to 350°F. Continue to bake for 15-20 minutes or until a toothpick inserted in the center comes out clean. Allow muffins to cool for 5 minutes in pan before transferring to a wire rack to cool.
Notes
All refrigerated items should be at room temperature, which helps the batter mix easily and evenly.
Measure flour accurately using the spoon and level method to prevent dense or dry muffins.
Even if you're not a coffee fan, adding espresso powder enhances the chocolate flavor without imparting a strong coffee taste. You can omit it if you don't have it.
Use an ice cream scoop to get an even amount of batter in each muffin tin.
It is important to bake at a higher temperature then lower the temperature which helps them rise but keeps them moist.
You'll know the muffins are done when a toothpick comes out clean.