Preheat the oven to 350F. Butter a 9-inch round cake pan, and line the bottom with a circle of parchment.
In a large bowl, beat the butter and sugar until light and fluffy, then beat in the eggs one at a time, beating after each addition. Fold in the almonds, cold mashed potato, lemon zest and baking powder. Gently stir in the berries.
Spread into the pan, smoothing the top, then bake for 40-45 mins or until golden and springy to the touch.