Fudgy dark chocolate sheet cake with mousse frosting

Ingredients

• 225 g dark chocolate (we used 62 per cent cocoa)
• 60 ml vegetable oil
• 2 eggs
• 1 cup brown sugar
• 3/4 cup plain flour
• 1/2 tsp baking soda
• 1/2 tsp baking powder
• 1/4 cup cocoa
• 1/4 tsp salt
• 2 tsp vanilla extract
• 1/3 cup maple syrup
• 1 cup ground almonds
• 1/4 cup milk
• 250 g (1 large) beetroot, peeled, finely grated, excess moisture squeezed out (see notes)
• Mousse frosting
• 220 g milk chocolate
• 220 g butter, room temperature
• 1 1/2 cups icing sugar
• 1 tbsp golden syrup