For the brownie, grease three 8in x 8 in (20cm x 20cm) baking pans with baking spray, then line the pans with parchment paper, leaving an overhang of paper above the tops of the pans to easily remove the brownies. Preheat the oven to 163°C (325°F).
In a small bowl, melt the butter, canola oil and the chocolate chips for 20-second intervals in the microwave, stirring after each interval, until the mixture is smooth, 1-2 minutes. Set aside the mixture to cool for a minute.
In a small bowl, whisk together the flour, cocoa powder and salt until they’re evenly distributed.
In a large bowl, whisk together the eggs, granulated and brown sugars until they’re just barely combined. Don’t overmix. Whisk the chocolate mixture into the egg mixture, then sift the dry ingredients over the wet, mixing just until the last streak of flour is combined.
Distribute the batter evenly among the prepared pans. Bake the brownies for about 35 minutes, until the centres look set. Remove the brownies from the oven and allow them to cool, in the pans, to room temperature.
For the caramel sauce, place the sugar and water in a medium, light coloured saucepan with a lid over medium heat. Allow the sugar to melt in the water but do not stir it. After about 10 minutes, when the colour of the sugar begins to turn amber, remove the pan from the stove and stir in the cream. The mixture will bubble violently, so be careful. Stir the mixture until the cream is completely combined, whisk in the butter and pour the caramel into a heatproof bowl.
Add the vanilla and salt, mixing until they’re completely combined. Cover the caramel and allow it to cool to room temperature before refrigerating it.
Make the three batches of the thick dark chocolate ganache.
To make the frosting, place the chocolate in a heatproof bowl. Heat the cream in a small saucepan over low-medium heat until it’s just simmering, not boiling, 1-2 minutes.
Pour the hot cream over the chocolate, cover the bowl, and let it stand for 1-2 minutes. Use a fork to slowly mix the chocolate, until it’s smooth and fully incorporated with the cream. Use the chocolate ganache as directed in the recipes, either as-is or whipped.
To whip the chocolate ganache, cover it and allow it to cool completely to room temperature. Once it’s completely cool, use an electric mixer to whip it at high speed for 2-3 minutes, scraping down the sides of the bowl as needed. The colour should lighten, and the chocolate frosting should hold stiff peaks.
Add any extra flavours, and mix until they are fully incorporated.
Use the overhanging parchment paper to lift the brownie layers from the pans. Put the first brownie layer on a flat surface, then use a piping bag to pipe a border of ganache on it. Fill in the border with a layer of caramel, as much as you can without it spilling out, and then refrigerate the cake layer for 20 minutes.
Repeat with the second layer of brownie and caramel, reserving a bit of the caramel for the top of the cake. Place the third layer on top, and refrigerate the cake for at least 1 hour, until the ganache feels firm and the cake feels stable. Then, frost the top of the cake with the rest of the ganache and drizzle the ganache with caramel.
This is best served slightly chilled. Store it in an airtight container in the fridge for up to a week.