1/2 cup (4 oz) unsalted butter, room temperature
1/3 cup confectioners' sugar
1 large egg yolk
2 tablespoons finely grated lemon zest plus 1/2 teaspoon lemon juice
1/8 teaspoon coarse salt
1/2 cup macadamia nuts, toasted and finely ground
1 cup all-purpose flour (spooned and leveled)
5 ounces bittersweet chocolate, chopped
Preheat oven to 350 degrees with racks in upper and lower thirds.
In a large bowl, using an electric mixer, beat butter and sugar on medium-high until light and fluffy, 2 minutes.
Add egg yolk, lemon zest and juice, and salt and beat until combined; beat in nuts. With mixer on low, add flour and beat just until combined, 2 minutes.
Form dough into a 4 1/2-inch-long log, wrap in plastic, and freeze until firm, 30 minutes. Cut into 1/4-inch slices, transfer to two parchment-lined baking sheets, and freeze 5 minutes.
Bake until golden at edges, 14 minutes, rotating sheets halfway through. Let cookies cool completely on wire racks.
Melt chocolate. Dip half of each cookie into chocolate; transfer to parchment-lined baking sheets. Chill until set, 10 minutes.