In a large bowl, sift together the flour, cocoa powder, baking powder, and salt. Set aside.
In a separate medium bowl, whisk together the eggs and sugar until well combined. Add the milk, oil (or melted butter), and vanilla extract. Pour the wet ingredients into the dry ingredients and stir just until combined and moistened. Do not overmix. Fold in the chocolate chips or chunks. Set the batter aside.
To make the chocolate ganache sauce, combine the chocolate and heavy cream in a medium heatproof bowl. Microwave in 20- to 30-second increments, stirring in between, until the chocolate is melted and the mixture is smooth. Alternatively, melt the chocolate over a double boiler. Set aside.
Heat a griddle or skillet over medium heat and coat with butter or oil. For each pancake, drop ¼ cup of batter onto the skillet. Cook for 1-2 minutes, until bubbles form on the surface and the bottom is set. Flip carefully and cook for another 1-2 minutes. Transfer to a plate and keep warm by loosely covering with aluminum foil if desired. Repeat with the remaining batter, greasing the skillet as needed.
Serve the pancakes immediately, drizzling them with the warm chocolate ganache sauce. Rewarm the sauce in the microwave for a few seconds if needed.