Preheat oven to 375. Grease a 9 inch round tart pan. To make crust, sift flour and cocoa into mixing bowl.Rub in butter with your fingertips til mixture resembles fine breadcrumbs. Stir in sugar and enough cold water to make a soft dough. Wrap dough in plastic wrap and let chill in refrigerator for 15 minutes.
Roll out dough on lightly floured surface and use to line the pan. Line with parchment paper and fill with pie weights. Bake for 15 minutes. Remove from oven and take out the paper and weights. Bake crust for another 10 minutes.
To make filling, beat butter and sugar together in bowl and gradually beat in eggs with cocoa. Melt chocolate in heatproof bowl set over pan of gently simmering water,then beat it into the mixture with the half and half.
Reduce oven temperature to 325. Pour filling into pie crust and bake for 45 minutes or til filling has set gently. Let pie cool completely, then transfer it to a serving plate. Cover with whipped cream. Decorate with chocolate flakes and curls. Chill til ready to serve.