I love adding a little whole wheat flour to sable dough as it adds a really welcome and subtle nuttiness — that being said, you can certainly replace it with all-purpose flour if you’d like. As for filling the cookies, while you can buy pistachio paste at specialty grocery stores, it’s easy (and cheaper) to make your own, and I’ve included the ingredients and method below. The dough you’ll make here will yield two 9-inch logs, and you’ll only use one for these cookies, so feel free to throw the other in the freezer to bake off at a future date, or double your batch of pistachio paste and bake the whole lot (which would yield 30 cookies).
Recipe slightly adapted from Bon Appetit
Make the Honey-Vanilla Sable Dough:
Whisk both flours, cardamom, and salt in a medium bowl to combine. Place sugar in a large bowl and scrape in vanilla seeds; save pods for another use. Massage mixture with your fingers until sugar looks like wet sand.
Add butter to the sugar mixture and beat with an electric mixer on medium speed until smooth. Add egg yolk and honey; beat to combine. Reduce speed to low; add dry ingredients in 2 additions, mixing well after each addition. Knead dough a few times to incorporate any dry bits in the bottom of the bowl.
Divide dough in half and pat into two 9″-inch logs. Wrap each log in parchment paper, then wrap tightly in plastic. Roll each log across work surface to make as round and regular as possible, then chill until firm, about 2 hours. Note: dough can be made 1 month ahead and frozen; thaw in refrigerator overnight before using.
Make the Pistachio Paste:
In the bowl of a food processor, process pistachios, honey and oil until a smooth paste forms (this could take a few minutes, and paste will be quite thick). Add the butter and process just until combined and smooth. Paste will be a little loose, so place in the refrigerator while baking cookies to help it thicken just a bit.
Make the Cookies:
Preheat the oven to 350 F. Place chopped pistachios in a small bowl. Unwrap dough and slice crosswise into 15 even pieces and roll each between the palm of your hands into smooth balls. Press gently into pistachios to coat half of each ball, the place, pistachio side up, on a parchment-lined baking sheet, spacing about 2″ apart.
Bake cookies until barely golden, 8-10 minutes. Remove from the oven and press handle of a wooden spoon gently into each cookie, about 1/2 way down, to make a round indentation. Then wiggle it a little to wide the indent (or use your thumb, which I found a little easier). Return to the oven; bake until golden brown, 5-8 minutes. Let cool on baking sheet.
Once cool, fill each indent with pistachio paste.
Serving / Storage Notes: Cookies are best served the day they’re made, but are just fine covered at room temperature for up to 1 day. Cookies can be baked (but not filled) up to 3 days in advance — basically, once you fill them, you want to serve them pretty soon thereafter.