1. Place gelatin in a bowl of cold water to soak for 5 minutes.
2. In a small saucepan, heat the heavy whipping cream.
3. Whisk in the lemon juice and vanilla.
4. Add gelatin and coloring, mixing until dissolved. Take off the heat.
5. In a mixer, beat the egg whites on the medium speed, increasing to high once the soft foam is formed. Beat until stiff peaks are achieved.
6. Add sugar, spoon by spoon.
7. When all the sugar is incorporated turn speed down a little and slowly pour in the cream mixture. At this point the foam will lose around 30% of its volume but that is fine.
8. Place the foam in a baking dish lined with plastic wrap, and let set for a minimum of 8 hours (gelatin reaches the maximum strength after 24 hours).
9. Once set, remove from the form by lifting plastic film and cut to the desired shape.
10. Melt 100g of bittersweet chocolate over a bain-marie or double boiler. Once melted take off the heat and whisk in remaining chocolate until incorporated.
11. Enrobe foam squares into the melted chocolate and place on a drying rack to set.
12. Store in the fridge for up to three days.