Preheat the oven to 180 celsius (350 Fahrenheit) and grease a 20cm (8 inch) cake tin and line the base and sides with baking paper.
Slice the firm persimmons. A great way to do this is with a mandoline to achieve even slices. Drizzle the maple syrup over the base of the cake tin and arrange the persimmon slices on top, in 2 layers, so that the base of the cake tin is covered.
Toast the walnuts in a dry pan over a low heat, tossing occasionally to make sure they don't burn. Remove from the pan and leave to cool. When cooled place in a food processor and process into a fine meal. Be careful not to take this too far otherwise you'll end up with walnut butter!
Scoop out the flesh from the 2 soft persimmons and place in a food processor along with the bourbon and puree until smooth.
In the bowl of an electric mixer cream the butter and sugar until pale and fluffy, around 10 minutes. Add the eggs, one at a time, beating well in between each addition, until well combined. Place the flour, baking soda, baking powder, spices and ground walnuts in a bowl and stir to combine. Switch the mixer to it's slowest speed and add half the flour mixture along with half the persimmon puree. When combined add the remaining flour mixture and persimmon puree and beat until the mixture just comes together.
Carefully spread the cake mixture over the top of the persimmons and smooth the top. Bake for 1 hour or until cooked. Remove from the oven and place on a cooling rack to cool for 20 minutes before removing the cake from the tin.