Muffins: Preheat oven to 400 degrees F. Grease or line 12 muffins cups.
Combine sugar and zest. Whisk in the flour, baking powder, baking soda and salt. In a small bowl, whisk the sour cream, eggs, vanilla, lemon juice and butter until well blended.
Pour the liquid ingredients over the dry ingredients and gently stir to blend. Don't overmix. Stir in the chocolate chips. Divide the batter evenly among the muffin cups.
Bake for 18-20 minutes, or until the tops are golden and a thin knife inserted into the center of the muffins comes out clean. Cool for 5 minutes before carefully removing each muffin from its mold. Cool the muffins completely on the rack before topping with ganache.
Ganache: Heat cream until it's near a boil. Pour over chocolate chips and let it sit for 1 minute. Stir until smooth. Cool until it's a good consistency to drizzle. Use a teaspoon to drizzle chocolate over the muffins or spoon it into a Ziplock bag and snip off the corner to drizzle. Serve.