Preheat the oven to 160°C/fan 140°C/gas mark 3. Grease and line a deep springform 18cm cake tin.
Heat the butter, sugar, treacle and golden syrup together in a small saucepan until everything is melted. Remove from the heat, stir through the milk and leave to cool.
Sift the flour, ground ginger, mixed spice and bicarbonate of soda together in a large mixing bowl. Stir everything together and make a well in the centre. Pour in the beaten egg and gently stir everything together. Finally, stir through the cooled butter mixture.
Pour the cake mixture into the prepared tin and bake for 45 minutes or until firm to the touch. Leave to cool in the tin for 10 minutes before turning out on to a wire rack to cool completely. Once cooled, gently cut the cake in half horizontally, trimming down slightly if necessary. Clean the cake tin, grease with oil and line with plenty of cling film hanging over.
Soak the gelatin leaves in a bowl of cold water for 5 minutes. Squeeze any excess water out of the gelatin. Gently warm the orange juice in a small saucepan over a low heat and add the gelatin. Swirl the pan until the gelatin has completely dissolved.
Whisk together the yogurt, stem ginger, honey and vanilla bean paste until combined. Pour in the orange and gelatin mixture and whisk again until combined. Place the bottom ginger sponge half back in the bottom of the lined cake tin, cut side up. Arrange the fig slices around the edge of the inside of the tin, cut side facing out. If the stalks reach above the tin, trim those down slightly.
Spoon the yogurt filling inside, smoothing the top over with a spatula. Add the second sponge layer, making sure the cut side is facing down. Wrap with the cling film hanging over and place in the fridge overnight to set. The next day, carefully release the cake from the tin. Dust the top of the cake liberally with the cocoa powder.