Melt chocolate in top of double boiler set over simmering water. Stir until smooth, remove from heat and cool until lukewarm. Preheat oven to 350°F. Place cupcake liners in two 12-cup muffin tins and one 6-cup muffin tin.
Stir flour, baking powder and salt in a large bowl until well mixed. Using an electric mixer, beat butter, sugar and vanilla until creamy and smooth. Add eggs one at a time, beating well after each. Stir in chocolate. On low speed, beat in 1/3 of dry ingredients, then half of milk. Repeat, ending with last third of dry ingredients. Do not overmix.
Fill each muffin cup about 2/3 full and bake until a toothpick inserted in center of a cupcake comes out clean, 15 to 20 minutes. Cool in pan on wire racks for 5 minutes, then remove and cool completely on racks.