Stir together the flour, sugar and yeast in a large bowl. Mix the salt, milk and eggs together in a jug then pour into the flour mixture and beat well using a wooden spoon for about 5 minutes to make a thick, sticky batter.
Gradually add the butter, beating until the mixture is smooth, elastic and shiny. Finally fold in the orange and lemon zest. Cover the bowl with cling film and leave to rise for 1 hour.
For the syrup, tip the sugar into a pan, add the lemon juice and 150ml/5fl oz water and bring to a simmer, stirring until the sugar dissolves. Remove from the heat, stir in the orange liqueur and leave to cool.
For the chocolate disc, temper the plain chocolate by melting three-quarters (75g/2½oz) of the chocolate over a pan of simmering water (do not let the bottom of the bowl touch the water). Stir until the chocolate reaches a melting temperature of 50C. Remove the bowl from the heat, add the remaining chocolate and stir until it’s cooled to 31C.
Spoon into a piping bag made out of baking parchment and pipe a 5cm/2in oval disc onto a sheet of baking paper or acetate and leave to set. Spoon the melted white chocolate into another piping bag and pipe the word ‘Savarin’ onto the plain chocolate disc.
For the caramel shards, line a baking tray with silicone or baking parchment. Add the sugar to a pan with 4 tablespoons water and bring to a simmer, stirring until the sugar dissolves. When all the sugar is dissolved, bring the syrup to a boil without stirring until it reaches 170C on a sugar thermometer (CAUTION: boiling sugar is extremely hot. Handle very carefully). Immediately pour out onto the lined tray and leave to harden. Crack with a spoon or cut into shards using a knife.
Grease a 23cm/9in bundt tin or savarin mould with butter. When the batter has risen, spoon it into the tin. Cover with oiled clingfilm and leave to rise for 45 minutes to 1 hour, until it reaches three-quarters of the way up the tin.
Preheat the oven to 180C/160C Fan/Gas 4.
remove the cling film and bake for 20–25 minutes or until the savarin is risen and golden-brown. Remove from the oven and place the tin on a wire rack to cool for 5–10 minutes.
When cool enough to handle, remove the savarin from the tin and pour half of the syrup into the tin. Gently place the savarin back into the tin to soak up the syrup and cool completely. Pour the remaining syrup into a roasting tin, then place the savarin into the syrup and leave to soak for 5 minutes. Carefully transfer to a serving plate.
Meanwhile, for the Chantilly cream, whip the cream, icing sugar and vanilla together until soft peaks form when the whisk is removed. Spoon one-third of the Chantilly cream into a piping bag fitted with a star nozzle. Set aside in the fridge until ready to serve.
Using a sharp knife, segment the zested orange. Carefully slice off the top and bottom of the orange. Using even downward strokes, slice the skin away from the flesh and discard. Remove any remaining white pith.
Pipe the Chantilly cream around the top of the savarin and arrange the orange slices over the cream. Fill the savarin with the remaining Chantilly cream and decorate with sliced fruit. Top with the chocolate disc and caramel shards.