Blackberry Linzer Cookies

Combine butter, sugar, egg, cream and vanilla.
Then add salt, baking powder and flour, and beat until just combined, forming a soft dough.
Form into flattened disk, cover, and refrigerate 1 hour.
Place dough between two large pieces of parchment paper, and roll out to 1/8 inch thickness.
Cut out as many 2-inch round cookies as you can, placing on a lined baking sheet.
Use a linzer cookie cutter or a 1-inch round cookie cutter to create center holes in half of the rounds.
Chill all dough you aren't working with, including all the cutting scraps, in the refrigerator before rolling them out again to make more rounds.
Chill cut-out dough 10-15 minutes, if possible, before assembling cookies, so cookies won't lose their shape when handling.
Preheat oven to 350 degrees F when ready to assemble and bake.
To assemble, fill a small piping bag (or a quart-size resealable plastic food storage bag with the tip of one of its bottom corners snipped off) with the jam.
Squeeze 1/2 teaspoon of the jam onto the center of each bottom round of dough.
Top with the second rounds that have a hole in them, pressing gently to create a sandwich.
Space cookies 1 inch apart and bake one sheet at a time for 10 to 12 minutes or until the edges are barely golden brown.
To make lemon-thyme sugar: Whisk together zest, sugar and thyme leaves.
While cookies are still hot on the baking sheet, immediately sprinkle them with lemon-thyme sugar, then transfer the cookies to a wire rack to cool completely.