For the key lime pie, first prepare the shortcrust pastry. To do this, cut the butter into small pieces and quickly knead all the ingredients into a dough. Let rest briefly in the refrigerator. Roll out the shortcrust pastry on a floured work surface and line a springform pan (approx. 24 cm diameter) with it, raising a rim. Pre-bake in the preheated oven at 175 °C for 8 minutes.
Separate three eggs for the lime filling. Place three egg yolks and one whole egg in a bowl and beat until foamy. Stir in condensed milk and lime zest. Now slowly stir in the lime juice, continuing to beat.
Beat the egg whites with the baking powder until firm and gently fold into the lime-yolk mixture.
Spread the filling on the pre-baked short pastry and bake at 175 °C for 20 minutes. Remove the finished lime cake and allow to cool.
For the topping, whip the cream with a little sugar until stiff and spread on the cake.
Serve the key lime pie cold.