Arabian nights coffee chocolate cake with rose-scented ganache

Name: Arabian nights coffee chocolate cake with rose-scented ganache
Ingredients: 90g unsalted butter, plus extra for greasing; 250g plain flour; 1 1⁄2 tsp bicarbonate of soda; 1⁄2 tsp fine sea salt; 1⁄2 tsp finely ground cardamom seeds; 200g dark chocolate (70-75% cocoa solids), grated or blitzed in a food processor; 120ml vegetable oil; 400g caster sugar; 4 large eggs, lightly beaten; 2 tsp vanilla extract; 240ml strong hot espresso; Edible rose petals, to decorate (optional); 200g dark chocolate (70% cocoa solids), chopped 250ml double cream; 20g caster sugar; 1⁄2 tsp rosewater; FOR THE COFFEE BUTTERCREAM; 150g unsalted butter, softened; 300g icing sugar, sifted; 1 tbsp instant coffee granules, dissolved in 1 tbsp boiling water