The butter must be at room temperature, so warm it in the microwave for a few seconds; it should keep its shape, but a finger should easily penetrate it;
Beat the softened butter and the sugar in the stand mixer provided with the whisk attachment at low speed. If the butter gathers in the center of the whisk, switch the stand mixer off and remove the butter with a spatula, switch the mixer on again and increase its speed. Actually, this is not good, it means the butter is still too cold. You’d better wait some minutes longer- you will be able to combine the ingredients better;
Add salt, baking powder, freshly grated lemon zest;
Add vanilla flavoring and whisk at high speed until lightened and fluffy;
Add the eggs one at a time and then the yolk with the mixer running;
When the first eggs has been wholly incorporated, add the second, and go on until the last one;
Make sure the eggs have been incorporated and then pour the milk (it must be at room temperature or lukewarm). Whisk until a smooth, thick, fluffy mixture.
In another bowl mix flour, potato flour and baking powder; sift them twice onto a sheet of parchment paper;
Stir the mix of flour into the egg compound a little at a time;
If you use a stand mixer, decrease the speed and add all together;
Increase to medium speed, stir until an even, compact batter. Switch off the mixer;
If you don’t use a mixer, combine with downward motions as you don’t want a deflated batter;
Carefully butter and flour a 20-24 cm (8-18 in) high sided baking pan; if you don’t have it, line the side of your ordinary baking pan with parchment paper to make the side higher; the paper will stick perfectly as you have generously buttered the pan. Your cake will get incredibly high and fluffy;
Gently pour the batter into; smooth the top but don’t stir it, not to deflate it;
Pre-heat the oven at 150°C (302°F) and bake for about 60-70 minutes. You might bake the cake at 180°C (356°F) for 50 minutes but a lower temperature favors the rising process;
Check for its doneness. Stick a toothpick into the center of the cake, if it comes out clean your cake is done. The surface should be golden.
Switch the oven off; let the cake rest with the half closed door for 5 minutes;
Take it out of the oven and let the cake cool down on a cooling rack;
When completely cool, (it takes about 2 hours) generously dust with powdered sugar.