Preheat the oven to 160℃ (150℃ fan)/320°F/gas mark 3. Grease and line a 20cm (8in) square cake tin with baking parchment. Ensure the parchments extends beyond the sides of the tin to aid the removal of the magic cake once baked.
Gently melt 125g butter in a small pan and heat 475ml milk in another pan until lukewarm.
Separate 4 eggs and place the whites in one large mixing bowl and the egg yolks in the other.
Whisk the egg whites until standing in stiff peaks, then whisk in the vinegar.
Next, whisk the egg yolks with 150g caster sugar until pale and thick. Whisk in 2 capfuls of orange extract. Then stir in the melted butter with a spatula and mix well.
Sieve 100g flour and 30g cocoa powder into the batter and mix together until combined. Pour a third of the lukewarm milk into the batter and mixed to combine, then gradually add the remaining milk until all of the milk is incorporated into the batter.
Carefully fold the whisked egg whites, a third at a time, into the batter, until just combined. Pour the batter into the prepared brownie tin and bake in the centre of the oven for about 50 minutes (check after 40 minutes). The cake is ready when just the tiniest of wobbles still remain when you shake the pan.
Remove from the oven and allow the cake to cool in the tin for at least 15 minutes. Use the parchment to carefully lift the magic cake from the tin. Place onto the cooling rack to cool completely
Serve cut into squares and lightly dust with icing sugar just before serving if desired.