Wheat Biscuit Shortbread

Arrange the racks in upper and lower thirds of the oven and preheat the oven to 325°F. Line 2 large cookie sheets and with parchment paper or silicone sheets.
In a large bowl, combine the butter, sugar, vanilla and salt at medium-low speed just until smooth. Add the flours and mix until just blended. Divide the dough in half and then shape, without over handling, into 2 disks.
On a lightly floured surface, roll 1 piece of dough out into a 13-inch, 1/4-inch-thick square. Using a ruler and a fluted pastry wheel or a large knife, trim the edges and cut into 2- by 1-inch rectangles. Transfer the cookies to the baking sheets, spacing 1 1/2 inches apart. Reroll the scraps and repeat with the remaining dough. Mark each cookie several times with tines of a fork and then chill the pans in the refrigerator for 10 minutes. Bake cookies for about 17 minutes, until golden and nearly firm in center. Let cool for 5 minutes on baking sheets and transfer to racks to cool completely.
Finely chop semisweet chocolate. Place half of the chocolate in a heatproof bowl set over a saucepan with 1 inch of simmering water and stir until the chocolate is melted and smooth. Add the remaining chocolate to the bowl; remove the bowl from the pot and place on a folded towel. Stir occasionally until chocolate is smooth. Scrape the chocolate into a small bowl. Line a baking sheet with clean parchment or waxed paper. Dip the cookies halfway into the chocolate, let the excess drip off, and then place on the paper-lined sheet; let stand until set and dry, at least 1 hour.