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Blend butter, sugar, vanilla, and salt in a bowl with a fork until combined well
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Sift flour, corn starch, and cocoa into butter mixture and blend with fork until mixture forms a soft dough
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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
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Chill dough, uncovered, until firm, about 30 minutes.
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Press into tart shells; prick or dock all over inside, ensuring sides are not too thin
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For miniature tarts: Refrigerate dough; taking small ball, pat by hand into miniature muffing tin cups
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Refrigerate 15 minutes before baking
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Bake in convection oven at 325°F for 7 minutes
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Freeze in air tight container until ready to use