Heat oven to 220C/200C fan/gas 7. Prick the potatoes all over with a fork, then bake whole for 1 hr 20 mins or until crisp on the outside and fluffy inside. Set aside on a wire rack to cool. Reduce oven to 180C/160C fan/gas 4.
Once the potatoes are cool enough to handle, cut in half and scoop out the flesh into a large bowl, discarding the skins. Mash the potato really well, then mix in two-thirds of the melted butter, all the flour and the bicarbonate of soda. Season well.
Line a large (approx 35 x 25cm) baking tray with a piece of baking parchment and brush with half the remaining butter. Roll out the potato dough on a floured surface, then lay it on the lined baking tray – don’t worry if it breaks, just push the pieces back together once on the tray. Brush the dough with the remaining melted butter and bake for 45 mins or until golden brown on the outside.
Remove from the oven and allow to cool on the tray. Meanwhile, put the cream cheese in a large bowl and whisk until smooth and light. Stir in the double cream and lemon juice, and season well with black pepper. Spread the cream cheese mixture all over the surface of the cooled potato cake. Use a large, sharp knife to cut the potato cake into 30 squares, then top with the strips of salmon, the dill fronds, caperberries and lemon zest. Transfer to a serving platter and serve.