Start by making the brownies. Preheat the oven to 180oC/Gas 4 and grease a 30x20cm/12x8in tin.
Melt the chocolate and butter in a bowl over a pan of simmering water. Stir gently until smooth then set aside to cool slightly. In a large bowl, whisk together the eggs and sugar until pale and fluffy. Whisk the chocolate and butter mix into the eggs then gently fold in the flour and chopped pecans until fully incorporated. Pour into the prepared tin, smooth the surface then bake in the oven for 25 minutes, until the sheet of brownies are firm in the rim but a bit soft in the middle. Leave to cool in the tin for 10 minutes, then place on a wire rack until cooled completely.
While the brownies are cooling make the chocolate mousse. Melt the chocolate in a bowl over a pan of simmering water. Once melted stir in the cocoa powder then remove from the heat and stir in the cream and orange blossom water. Whisk the egg whites in a clean bowl, and half way through whisk in the sugar bit by bit, continue to whisk until stiff. Then gently fold them into the cooled chocolate mixture.
Pour the orange blossom water into the cream and whisk together, until softly whipped.
Assemble the trifle. Break pieces of the cooled chocolate pecan brownie and add half of them to your trifle bowl, to create the first layer. Then spoon half of the chocolate mousse on top, followed by the orange blossom cream, repeat with another layer and sprinkle the chopped pecans and chocolate curls on top.