Heat oven to 180C/160C fan/gas 4. Grease 2 x 20cm sandwich tins and line the bases with baking parchment. Melt butter, sugar and maple syrup in a pan over gentle heat, then cool slightly. Whisk the eggs into this mixture, then stir in the flour, baking powder and mixed spice, followed by the grated parsnip, apple, chopped pecans, orange zest and juice. Divide between the tins, then bake for 25-30 mins until the tops spring back when pressed lightly.
Cool the cakes slightly in the tins before turning out onto wire racks to cool completely. Just before serving, mix together the mascarpone and maple syrup. Spread over one cake and sandwich with the other. Dust with icing sugar just before serving.