Walnut Cookies With Ganache Filling

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   ["Cream sugar and butter.", "Add vanilla, lemon zest, flour and ground walnuts.", "Mix on medium speed, until just combined.", "Remove the dough from the bowl and flatten it to a round disk.", "Wrap it loosely in plastic and chill for 1 hour.", "Heat the oven to 325 degrees.", "Roll the chilled dough into a large rectangle, 1/4"" thick.", "Cut into 2x2"" squares onto a nonstick baking sheet.", "Bake until light brown around the edges 10 to 14 minutes.", "Cool, then dust with cocoa powder.", "While the cookies cool, bring the heavy cream to a boil in a pot over high heat.", "Put the chocolate pieces in a large bowl, and when the cream comes to a boil, pour it over the chocolate.", "Stir until completely melted and smooth.", "Cool chocolate mixture until slightly thickened.", "Using a pastry bag, fill the bag with ganache, pipe a small circle on top of one cookie, and top with another to create a sandwich."]

* Per serving · 4 servings · Values may vary.