Preheat the oven to 170°C (338°F) and line a muffin tin with paper liners. Mix Dry Ingredients: Sift together flour, baking powder, sugar, and salt in a large bowl. Combine Wet Ingredients: Whisk cocoa powder with warm milk, then add eggs, vanilla, and oil. Mix Together: Pour wet ingredients into dry, mixing until just combined. Fold in chocolate chips. Fill Muffin Cups: Divide batter into muffin cups, filling each two-thirds full. Top with extra chocolate chips. Bake for about 20 minutes until a toothpick comes out clean. Make Ganache: Melt dark chocolate with heavy cream. Stir until smooth. Cool and Fill: Cool muffins and fill or drizzle with ganache.