For the chocolate pastry, beat the butter & icing sugar until smooth. Now add in egg yolk & combine well. In a separate bowl, sift together flour, cocoa powder, corn starch & salt.
Mix all dry ingredients with the butter mixture and make smooth dough. Keep it in the chiller for an hour.
Remove from the chiller and roll it carefully with the help of dry flour dusting, Spread carefully in tart pan and adjust all the corners with the help of forefinger & thumb.
Prick the center with the fork and cover it with baking sheet & pie-weight. Bake the tart shell for 15 minutes at 180 c. Then take it out from the oven and remove pie-weight.
Put it back in the oven for further 15 minutes to give the perfect color. Use it accordingly.