Make the filling: In a small saucepan over medium heat, whisk egg yolks, sugar, and lemon juice until blended. Add butter and lemon zest. Cook, stirring constantly, until mixture is thickened, around 15 minutes. Remove from heat and add 1 tablespoon potato flour. Stir until well combined and set aside.
Make the crusts: Preheat the oven to 200°c. Brush the wells of a 12-count non-stick mini tart pan lightly with vegetable oil. Divide the pastry into 12 equal parts and form little balls using your hands. Press the dough into each of the pans in an even layer. With a fork, poke a few holes in the bottom of the crusts. Bake for 5 minutes.
Remove from the oven and add 1-2 tablespoons of lemon filling to each mini pie crust. Bake for 10 minutes. Remove from the oven and reduce heat to 100°c.
Make the meringues: Beat the egg whites using an electric mixer. Start on slow and then gradually add the icing sugar. Increase the speed to high and beat for 5-7 minutes. When the meringue is white and forms semi-stiff peaks, add 1 tablespoon potato starch and beat for 1 minute. The mixture is ready when it forms stiff peaks.
Line a large baking tray with parchment paper. Fit a large disposable piping bag with a medium tip and transfer meringue mixture to the piping bag. Pipe the mixture onto the baking tray. Bake for 1 hour. Turn off the oven, but do not open it. Let the cookies cool completely in the oven (1hour) before removing.
Just before serving, decorate the mini lemon pies with meringue cookies.
