Break the orange jelly into cubes and dissolve in 150ml boiling water (half the packet instructions to make it firmer). Stir in the finely grated orange zest. Pour into a shallow container lined with cling film and refrigerate for at least 2 hours until fully set.
Preheat the oven to 180°C (350°F). Grease a 12-hole shallow bun tin with butter.
Whisk the eggs and caster sugar together with an electric whisk until thick, pale, and the mixture leaves a trail when the whisk is lifted — about 5 minutes.
Gently fold in the sifted self-raising flour using a metal spoon, being careful not to knock out the air.
Divide the sponge batter evenly between the greased tin holes. Bake for 8-10 minutes until golden and springy to the touch. Turn out onto a wire rack and allow to cool completely.
Once the jelly is set, use a round cutter (roughly the same size as the sponge bases) to cut out jelly discs. Place one jelly disc on top of each cooled sponge.
Melt the chocolate in a heatproof bowl set over a pan of barely simmering water, stirring until smooth.
Spoon the melted chocolate over each cake, covering the jelly and letting it drip slightly down the sides. Top with chopped pistachios, small pieces of candied orange, and a pinch of sea salt.
Allow the chocolate to set at room temperature or refrigerate briefly. Serve and enjoy.