Preheat the oven to 340°F (170°C). Prepare the muffin pan with paper liners.
In a medium mixing bowl, add butter, sugar, and salt, and mix together until well blended and creamy. Add one egg at a time, and mix in.
Sift in flour, cocoa powder, and baking powder, and mix until just combined. Mix in milk and vanilla extract into the batter.
Add chopped semi-sweet chocolate chips, and fold them into the batter, being careful not to overmix.
Spoon the batter into the muffin liners, to about 3/4 full. Top with extra semi-sweet chocolate pieces. Bake for about 25 minutes, just until a toothpick in the center comes out clean. Every oven is different, so I recommend checking the muffins about 20 minutes into the baking time to prevent overbaking.
Transfer the muffins to a wire rack to cool. Enjoy!