In a medium bowl, whisk to combine egg yolks, sugar and salt until thickened. In a small saucepan, heat 1/4 cup heavy cream over medium until bubbles start to form around the edges of the pan. Stir in chocolate and melt completely.
Whisk 1/3 of the chocolate into the egg yolk mixture, then add back to pot with remaining melted chocolate. Cook over medium heat, stirring often, for 2 minutes. Remove from heat and set aside to cool slightly.
In the bowl of a stand mixer with whisk attachment, whisk remaining 1 cup heavy cream to medium peaks. Fold cooled chocolate mixture into cream until completely combined and transfer to a pastry bag fitted with a 3/4-inch tip. Refrigerate for 2 hours until completely chilled.
For the Brownies:
Preheat oven to 350 degrees F. Spray an 8x8-inch square baking pan nonstick baking spray and line pan with a piece of parchment paper, leaving the edge of the paper overhanging slightly. This will make it easier to remove the brownies from the pan once they are cooked and cooled.
Melt the oil in a small saucepan over a medium heat, stirring gently. Once completely melted, remove from the heat and add the dark chocolate. Stir until fully combined and melted.
In a large bowl, whisk to combine the coconut flour, baking powder, salt, and cocoa powder. Place the eggs in the bowl of a stand mixer fitted with the whisk attachment, or use an electric whisk, and beat until light and creamy. Alternatively you can do this by hand using a bit of elbow grease! Add the sugar and continue to beat until all is incorporated.
Gradually add the melted chocolate and oil mixture to the egg and sugar mixture, whisking the whole time. Gently fold in the coconut flour mixture, making sure everything is well incorporated, but trying not to knock out any air.
Spoon batter into prepared pan, level out and dust with an extra sprinkle of sea salt.
Bake for about 25-30 minutes, or until a skewer inserted around the outside of the cake comes out clean. The very middle can still be pretty squidgy – you want to take it out when it’s slightly underbaked, so that it stays super-gooey and moist. Cool in the tray before cutting.
For the coconut moss.
Line a baking sheet with parchment paper. In a medium bowl, combine coconut and food coloring gel until coconut is a light green “moss” color. Spread onto prepared baking sheet and let dry.
Fill a large piping bag with chocolate mousse. Pipe mousse into bottom of terrarium and use a spoon or offset spatula to smooth and fill in the gaps around the edges.
Top mousse with candy rocks, then graham crackers, crumbled brownies and coconut moss. Place cacti in terrarium to finish.