To make the pastry, preheat the oven to 200C/180C Fan/Gas 6.
Rub the butter, sugar and flour together in a large bowl or mix in a stand mixer with a paddle attachment. Add the egg and cream. Bring together to form a dough and shape into a ball with your hands.
Lightly flour a work surface and roll out the dough into a circle about the thickness of a pound coin. Line a 23cm/9in loose-bottomed tart tin with the pastry, pushing into the edges and trimming off any excess. Bake for 20–25 minutes or until cooked and golden. Leave to cool and leave the oven on.
To make the filling, whisk the eggs and sugar for 5 minutes in a stand mixer with a whisk attachment or an electric hand whisk, until tripled in volume. Place the butter, chocolate and salt in a glass bowl and melt over a saucepan of gently simmering water. Fold the chocolate mixture into the egg mixture and spoon into the pastry shell. Bake for 15 minutes then leave to cool and set.
To serve, gently whip the cream with the cinnamon in a large bowl until just forming soft peaks. Top the tart with the cinnamon cream and sprinkle over the chocolate shavings. Cut into slices and serve.