Preheat the oven to 200°C (180°C Fan oven, 400°F, gas 6). In a bowl, whisk the eggs and caster sugar together until thick, pale and creamy – this will take about 5 minutes.
Add the Vanilla and sift the flour on top. Using a large spoon, fold the flour into the egg mixture.
Line a baking tray with baking parchment, and mark out a 20cm (8inch) square.
Pile the sponge mix on top of the square and gently spread to cover. Bake for 8 -10 minutes until risen, golden and just firm to the touch. Cool for 10 minutes then transfer to a wire rack to cool on the paper.
Measure the tin against the sponge and carefully trim away sufficient crust from the sponge so that it fits snugly inside the bottom of the tin.
Line an 18cm (7inch) square tin with a double layer of cling film. Press the sponge inside the tin and set aside.
Pour 150ml (1/4 pt) hot water into a large heatproof jug and stir in the gelatine until it dissolves. Stir in the sugar and orange juice. (NOTE: If you're using Vege-Gel like I did, you'll need to stir in the orange juice and bring it to the boil in a pan for adding the sugar and continuing with the rest of the recipe)
Leave to cool until just on the point of setting, then pour or spoon over the sponge base. Chill for at least 2 hours to set firmly.
Break the Extra Dark and Milk Chocolates into pieces and place in a saucepan with the butter and 30ml (2 tbsp) water. Heat very gently, stirring occasionally until melted together – take care not to boil the mixture. Remove from the heat, mix well, and cool for 10 minutes. Spoon the chocolate over the set orange jelly.
Leave in a cool place to firm up. Before the topping sets completely, score the chocolate with a sharp knife to make 16 equal portions. Once the chocolate topping becomes chilled, it will set very firm and will become difficult to cut if it is not scored first.
To serve, carefully lift out the jelly mixture from the tin using the cling film. Discard the film and cut the jelly squares completely through into portions.
Your chocolate jelly squares are now ready to serve and enjoy!