Position oven rack on middle rung. Heat oven to 350*. Lightly grease the bottom and sides of a 9 1/2" tart pan with a removable bottom.
In a large bowl, combine the butter, dark brown sugar and salt. Beat with an electric mixer on medium speed until well blended. Add the egg yolk and vanilla and beat just until combined. Pour in the flour and beat on low speed until the dough begins to clump together. Scrape the dough into the prepared pan, scattering the pieces evenly. Using your fingertips (lightly floured if necessary), pat the dough into the bottom (not up the sides) of the pan to form an even layer. Bake until the top looks dry and dough begins to pull away from the sides of the pan, 25 to 27 minute
Meanwhile, melt the chocolate with the cream in a double boiler or in the microwave. Stir until smooth. When the crust is baked, transfer the pan to a rack. Pour the warm ganache (chocolate cream mixture) over the warm crust and spread evenly to within 1/2 inch of the edge. Scatter the pecans evenly over the ganache and gently press them into the chocolate. Let cool completely until the chocolate is set, about 4 hours at room temperature or 2 hours in the refridgerator. Remove the outer ring of the tart pan and cut the "cookie tart" into 16 wedges. Serve chilled or at room temperature.
Big Batch Tip: Double all ingredients, following the same directions and timing, and bake in a 9x13" metal pan. Once cooled, cut into about 6 dozen bars.