Prep: Preheat oven to 170°C (340°F). Grease and line an 8-inch cake tin.
Melt Chocolate: Melt dark chocolate using a double boiler or microwave in short bursts. Let it cool slightly.
Creaming: In a large bowl, beat butter and sugar until light and fluffy (about 4-5 minutes).
Eggs: Add eggs one by one, beating well after each addition.
Add Chocolate & Vanilla: Mix in melted chocolate and vanilla.
Dry Ingredients: Sift together flour, cocoa powder, baking powder, and salt. Add to the wet mix in batches, alternating with milk.
Mix: Fold gently until just combined. Do not overmix.
Bake: Pour into the prepared tin and bake for 40-45 minutes or until a skewer comes out clean.
Cool: Let the cake cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.