Blend butter, sugar, vanilla, and salt in a bowl with a fork until combined well

Sift flour, corn starch, and cocoa into butter mixture and blend with fork until mixture forms a soft dough

For 2 medium tarts: Divide dough in half and pat out with floured fingertips into 2 (6 to 6 ½ inch) rounds on an ungreased large baking sheet

Chill dough, uncovered, until firm, about 30 minutes.

Press into tart shells; prick or dock all over inside, ensuring sides are not too thin

For miniature tarts: Refrigerate dough; taking small ball, pat by hand into miniature muffing tin cups

Refrigerate 15 minutes before baking

Bake in convection oven at 325°F for 7 minutes

Freeze in air tight container until ready to use