Preheat an oven to 180 C / Gas 4. Grease and flour a 26cm round springform tin.
Use an electric mixer to beat the eggs, apple sauce, black treacle and butter in a large bowl. Beat in the sponge cake mix along with the cinnamon, nutmeg, cloves and orange zest. Mix on medium speed for 4 minutes. Pour batter into prepared tin. Top the batter with pear slices; sprinkle evenly with the pecans and chopped chocolate.
Bake in preheated oven until a skewer inserted in the centre comes out clean, about 55 to 60 minutes. Cool for 25 minutes before removing from tin.
If Bosc pears are unavailable, substitute any ripe pear you prefer. You can also use tinned pears, if desired.