Place 150 grams of the chopped chocolate and the vegetable oil in a heat-proof bowl set over a saucepan of simmering water, ensuring the bottom of the bowl does not touch the water. Stir until fully melted and smooth.
Use a pastry brush to coat the interior of your silicone molds with a thin, even layer of the melted chocolate. Place the molds in the freezer for 10 minutes to set.
Apply a second thin layer of chocolate to the molds to ensure there are no thin spots or holes. Freeze again for 15 minutes until completely firm.
In a separate bowl, whisk the heavy cream, sugar, and citrus zest until soft peaks form.
Slowly drizzle the citrus juice into the whipped cream while continuing to whisk on low speed until the mixture thickens into stiff, glossy peaks.
Carefully remove the hardened chocolate shells from the molds by gently peeling the silicone away.
Spoon or pipe the citrus filling into each chocolate shell until level with the top.
Melt the remaining 50 grams of chocolate and drizzle it over the top of the mousse to create a decorative seal. Refrigerate for at least 30 minutes before serving.