Heat oven to 375 degrees Fahrenheit or 190 degrees Celsius.
In a bowl, mix flour, salt and baking powder
In another bowl, cream softened butter until fluffy, add sugar and blend well. Add egg and mix well. Add vanilla extract, water and zest of 1 lemon. Add chocolate and 1/2 of flour mixture. Mix well and add remaining flour. Mix well.
Butter an 8 inch square baking pan. Line with parchment paper overhanging on 2 sides. Butter parchment paper bottom.
Put mixture in pan and even out with a spatula. Bake in the heated oven for 20-25 minutes until cooked. Cool completely in the pan. Pull up on parchment paper and transfer to cutting board. Cut into 16 squares. Sprinkle with powdered sugar and enjoy!