Preheat the oven to 180C (350F) and line a 20cm/8in square tin with baking paper.
Peel and grate the potato really finely and then squeeze some of the moisture out. Set aside.
Whisk the eggs and Canderel Sugarly together for 3-4 minutes at maximum speed until pale and double in volume.
Whisk in the grated potato until well combined.
Add the ground almonds, flour, baking powder, vanilla and salt and whisk until incorporated.
Fold in half the blueberries and pour the batter into the prepared tin.
Dot the remaining blueberries over the top, pushing them gently halfway into the batter. Sprinkle with the flaked almonds.
Bake for 30 minutes. Lift the cake out of the tin and transfer to a cooling rack.
Warm the jam and water in the microwave for 30 seconds and stir together. Carefully brush over the cake to glaze.
Cut into squares and serve. Keep any leftovers in an airtight container and consume within a day.