In a medium bowl combine flour, salt and baking powder.
Place the butter in a small bowl over a pan of simmering water. Whisk until almost melted. Remove from heat and whisk until melted completely.
Transfer the butter to a large bowl. Add sugar and whisk shortly with a hand whisk. Add egg, vanilla, lemon zest and lemon extract and whisk until combined. Gradually add the flour mixture, mixing gently with a wooden spoon until just absorbed. Add the white chocolate pieces and mix shortly to distribute them around the dough.
Leave the dough in the bowl and cover with cling film. Refrigerate for 30-60 minutes.
Preheat oven to 180 degrees C (350 degrees F). Place the oven shelf on the top position in the oven.
Line 2 large baking tins with parchment paper.
With your hands make balls out of the dough and arrange them on the prepared tin. Leave a reasonable distance between the cookies, depending on their size.
Bake for 8-10 minutes or until the cookies start to get brown on the edges.
Let them cool on the tin for 5-10 minutes (they will be too soft and fragile when hot).
Transfer with a spatula to a wire rack to cool completely.