In a large bowl, cream together butter and icing sugar, then slowly mix in flour and baking powder.
Press into a 9" tart pan.
Bake for 10-12 minutes, or until edges are slightly browned.
Set aside to cool in pan.
While the crust is cooling off combine the lemon juice and zest in a small saucepan over medium-high heat, stirring occasionally.
Meanwhile, in a large heat-proof bowl, whisk together the egg and sugar.
Once the mixture in the saucepan comes to a boil remove from the heat and slowly pour it into the egg mixture, whisking continuously to temper the eggs.
Pour the mixture back into the saucepan and heat over low heat, whisking continuously until the mixture thickens to the point where it can coat the back of a spoon, about 10 minutes, then remove from heat and allow to cool slightly for 10 minutes.
Stir in the butter, one cube at a time, until smooth.
Pour the lemon filling into the crust and smooth out the top with a spatula.
Refrigerate until set, about 2 hours, prior to serving.