In a bowl, combine the flour, cocoa powder, and salt. Sift and set aside.
With an electric mixer, beat the butter for two minutes, then add sugar and vanilla and beat on high for one minute more.
Add the flour mixture slowly to the butter, beating on low until just combined. Dough will be crumbly.
Line a 10x15 inch rimmed baking sheet or jelly roll pan with aluminum foil, then transfer the dough into the pan. Press the dough so it's evenly distributed on the pan to the edges.
Score the cookie shapes into the dough. Small rectangles are best for these. Use a fork to prick lots of holes in the dough.
Bake 24-26 minutes, rotating halfway through. After a few minutes, recut along the same lines but don't remove from the tray.
While the cookies cool, make the ganache: combine cream and chocolate chips in a bowl and microwave, 20 seconds at a time until chips are nearly melted and stir to combine.
When cookies are fully cooled, remove from the pan and dip into the chocolate. Lay each cookie on a cooling rack to let the chocolate set.