For the Dough: Sift dry ingredients. Cream butter and sugar until fluffy, then add egg. Add dry ingredients until combined. Shape into a ball, cut in half, wrap in plastic and chill one hour (extra dough half can be frozen.) Roll out dough to 13-inch circle and chill one hour. Fit into 9-1/2 inch tart pan (removable bottom works well) and freeze 15 minutes. Bake at 350F for 20 minutes until the dough puffs up. Flatten the bottom with a measuring cup, pressing all around. Bake another 20 minutes until crisp. Cool.
For the custard: Preheat oven to 250°F Heat cream, milk and sugar. Pour over chocolate 1/3rd at a time, stirring. Beat egg in a separate bowl and whisk in 1/4 of the cream mixture. Whisk egg mix into remaining cream mix. Tap to get rid of extra bubbles. Pour into shell. Bake 40 minutes, rotating pan once. Custard should be set at edges and still jiggly in the center.