Preparation

Make the shortbread:

• Position a rack in the middle of the oven and heat the oven to 350°F. Lightly butter the bottom and sides of a 9-1/2-inch fluted tart pan with a removable bottom.
• In a medium bowl, combine the butter, sugar, cocoa, and salt. Beat with an electric mixer on medium speed until well blended. Scrape the bowl. Add the egg yolk and vanilla and continue beating on medium speed until just combined. Add the flour and mix on low speed, scraping the bowl as needed, until the flour mixes in and the dough begins to clump together, about 1 min. Scrape the dough into the pan, scattering the pieces of dough evenly. Using your fingertips (lightly floured, if necessary), pat the dough onto the bottom (not up the sides) of the prepared pan to create an even layer. Bake until the top no longer looks wet and the dough just barely begins to pull away slightly from the sides of the pan, about 25 min.

Make the glaze:

• Shortly before the shortbread is done, melt the chocolate and butter in the top of a double boiler or in a microwave oven. Stir until smooth. When the shortbread is done, transfer the pan to a rack. Pour the warm glaze over the shortbread and, using an offset spatula, spread the glaze evenly to within 1/2 inch of the edge. Scatter the nuts evenly over the glaze and gently press them into the glaze. Let cool completely until the glaze is set. Remove the shortbread from the tart pan and cut it into 12 or 16 wedges. Serve at room temperature.