1.Preheat oven to 200°C (180°C fan forced). Grease oven trays, line with baking paper.
2.Beat butter, sugar and egg in a medium bowl with an electric mixer until light and fluffy. Stir in buttermilk, treacle, coarsely grated apple, sifted flours and soda.
3.Drop level tablespoons of mixture about 5cm (2 inches) apart onto trays, tap trays lightly on the bench to spread slightly.
4.Bake pies about 10 minutes. Cool on trays.
5.Meanwhile, beat cream in a small bowl with an electric mixer until firm peaks form.
6.Sandwich pies with cream and caramel. Whoopie pies are best eaten the day they are made.