For the cupcakes: Preheat oven to 200C (400F) . Line a 12-cup muffin tin with cupcake liners.
Melt butter and 100g coarsely chopped dark chocolate in a heatproof bowl over a pan of simmering water. Stir until melted and smooth. Cool.
Sift flour, cocoa powder, sugar and salt into a large bowl.
In a small bowl, whisk together melted butter and chocolate mixture, eggs, milk and vanilla extract until thoroughly combined. Pour into flour mixture along with 100g chopped dark chocolate. Stir gently, just until combined.
Divide batter evenly into cupcake liners and bake 15-20 minutes or until the tops spring back when touched.
For the buttercream: Beat butter with a mixer on medium-high speed until pale and creamy, about 2 minutes.
Reduce speed to medium. Add sugar, ¼ cup at a time, beating after each addition, about 5 minutes. (After every 2 additions, increase speed to high, and beat for 10 seconds, then reduce speed to medium-high). Add vanilla, and beat until buttercream is smooth. Use immediately, or cover and refrigerate for up to 3 days (bring to room temperature, and beat on low speed until smooth before using).