Step 1

This filling can be made days in advance and refrigerated.

Step 2

Place the apricot preserves in the bowl of a processor. Process until smooth.

Step 3

Dissolve the gelatin in the vinegar in a very small bowl.

Step 4

Heat the preserves just to a simmer. Remove from the heat. Tear the gelatin into about 6 pieces and immediately add it to the hot preserves. There is no need to liquify it first. The heat will do that. Wait for a minute or two until you can see the gelatin has liquified. Stir it into the preserves. Transfer to a small container and refrigerate until needed.

Step 5

Preheat the oven to 325°F. Line a couple of baking sheets with parchment paper and set aside.

Step 6

Place the walnuts on a rimmed baking sheet and toast for 7 to 10 minuts until fragrant and slightly browned. Cool completely.

Step 7

When cool, place the walnuts and flour in the bowl of a processor. Process until the nuts are finely ground and instinguishable. Set aside.

Step 8

Place the butter and sugar in the bowl of a mixer. Beat until light.

Step 9

Add the egg yolk and vanilla and beat to combine.

Step 10

Add the flour mixture and beat on low just until completely incorporated. The dough should be good to roll out but if it is too soft, refrigerate briefly. Do not let it get too cold or it will be hard to roll.

Step 11

Place the dough between two large pieces of waxed paper. Make 4 or 5 indentations and then roll the dough to abut 1/4" thick. With a 1 1/2" round cutter, cut the cookies out, keeping them close together

Step 12

Remove the top piece of waxed paper and, keeping the cookies on the bottom pieces of paper, transfer the whole thing to a baking sheet. Freeze until rock hard.

Step 13

When frozen, simply punch them out and place them on a cookie sheet about 5 across and 7 down. Refreeze if necessary.

Step 14

Bake for 15 to 17 minutes from the frozen state until lightly browned and firm. Cool completely. These can be stored in an airtight container before filling for up to a week. The cookies can also be frozen, unbaked, wrapped well and kept for several months. Bake from the frozen state.

Step 15

The cookies can be filled or left as single cookies. If sandwiching continue.

Step 16

Sift the powdered sugar into a medium size bowl. Set aside.

Step 17

Stir the filling vigorously to smooth it out. Turn half the cookies upside down and fill with about 3/4 teaspoon apricot filling. Place the tops on the cookies.

Step 18

Allow the filled cookie to set up on a rack overnight to tighten the filling.

Step 19

If finishing with powdered sugar, place a cookie in the powdered sugar, coating both sides and rolling the edges in the sugar. Dust the tops with more powdered sugar.

Step 20

Store in an airtight container for a week or 10 days.