Preheat oven to 180°C. Line the base and sides of an 18cm x 28cm rectangular pan with baking paper.
Place butter, chocolate and ¼ cup cocoa in a heatproof bowl set over a pan of gently simmering water (don’t let bowl touch water), stirring until smooth. Leave to cool.
Whisk oil, VEGEMITE, brown sugar, 6 eggs and vanilla in a bowl to combine. Fold in flour and baking powder, then fold in chocolate mixture.
Spread batter into prepared pan.
Whisk 3 egg whites in an electric mixer with the caster sugar for 8 minutes until stiff and glossy. Whisk in 2 tbsp cocoa until combined.
Spread meringue over brownie mixture and bake for 45 minutes until meringue top is crisp. Cool brownies completely in pan.
For the brown buttercream, melt butter in a frying pan over medium-high heat for 2-3 minutes until brown.
Pour into a bowl and cool to room temperature, then whisk in the cream.
Chill until needed and whisk lightly before serving.
Cut brownie into 8 bars and drizzle with cream, to serve.