Preheat the oven to 180oC/fan 160°C/gas 4.
Generously butter a 23cm springform cake tin and lightly dust with the flour. Turn the pan upside down and bang out the excess flour. Wrap a large square of heavy-duty foil around the outside of the base of the cake tin.
Melt the butter in a small saucepan and stir in the orangello.
Create a Bain Marie (medium size ceramic mixing bowl fitted over a saucepan of simmering water). Please note: the base of the bowl must not touch the water. Put about 6cm of water in the saucepan and bring it to a simmer.
Add the chocolate to the bowl and pour over the melted butter. Stir from time to time until the chocolate is completely melted, then remove the bowl from the water.
Beat the eggs and sugar with an electric mixer on high speed for a full 10 minutes. They should quadruple in volume and become light coloured, very thick and fluffy.
Fold one-quarter of the egg mixture into the chocolate mixture and then very gently fold in the remaining egg mixture, orange zest and amaretto until incorporated. Pour the cake batter into the prepared pan.
Sit the cake tin into a baking tray or small roasting tin, to form a bain-marie. Place the baking pan with the cake on the centre oven rack and pour in enough water to come about 1/2 inch up the sides of the cake pan.
Bake for 25 to 30 minutes until the top of the cake will lose its glossiness and be slightly mounded, but it should not bake so long that it rises and cracks. If you insert a skewer into the centre it should come out gooey.
Let the cake cool completely in its pan on a wire rack. Run a small palette knife around the edge of the cake and remove the outer ring.
The cake will keep tightly covered in the refrigerator for up to 3 days. Bring it to room temperature before serving.
To make the sauce, place blackberries in a small saucepan; mash gently with a fork. Stir sugar, schnapps, and lemon juice into mashed blackberries; cook, stirring constantly, over medium heat until blackberries break down and mixture begins to boil, 5 to 10 minutes. Cool slightly before serving.