In a large mixing bowl beat the butter and the shortening with an electric mixer then add the sugar, powdered sugar, baking soda, cream of tartar and the salt. Beat just until combined.
Add the egg and vanilla, then mix in the flour gradually. Cover the dough and refrigerate for 3 hours.
Preheat the oven to 375F.
Roll the dough into 3/4 inch balls and put them on an ugreased cookie sheet. With your thumb or the end of a wooden spoon, press a circular indentation into the center of each cookie.(I like to use my cookie gun)
Drop a 1/4 teaspoon of jam into each thumbprint.
Bake cookies for 8-10 minutes. Then cool on a wire rack for 5 minutes.
Put the white chocolate chips in a microwave safe bowl and microwave on high in 30 second bursts, stirring each time until the chips are melted. (I like to use microwave safe disposable decorating bags by Wilton) Pour the white chocolate into a disposable decorating bar, snip off a tiny bit of one corner. Drizzle the white chocolate over cookies.