Iced Lemon Shortbread Hearts
For the cookies: Combine the flour, baking powder, sugar, vanilla sugar and lemon zest in a large bowl. Add the cold water, egg and butter and mix everything together using a pastry cutter or two forks. Quickly knead with hands to form a smooth dough. Wrap with plastic wrap and refrigerate for 30 minutes.
Roll the dough out on a floured surface to about 2-3 mm thick. Cut into heart shapes using a cookie cutter and place on a baking sheet lined with parchment paper. Bake in an oven preheated to 200°C/180°C convection/gas mark 3-4 (approximately 400°F/350°F) for 12-15 minutes, until golden brown. Remove from the oven and cool on a wire rack.
For the icing: Sift the powdered sugar into a bowl and mix with the lemon juice until smooth. Spread the icing onto the cooled cookies and allow to dry.