Preheat oven to 180°C (160°C fan) / 350°F. Line the tin with greaseproof paper.
Melt chocolate and butter until smooth. Cool slightly.
Whisk eggs, sugar, molasses, and vanilla until thick and glossy. Mix in chocolate.
Sift and fold in flour, cocoa, spices, salt, and baking powder. Add chocolate chunks if using.
Pour into tin, smooth top, and bake 25-30 minutes until edges are set. Cool completely.
Drizzle melted chocolate over brownies and allow to set. Decorate if desired. Slice into squares.