Sable cookies are a French classic. This dark-and-white chocolate version from Umber Ahmad of Mah-Ze-Dar bakery is just the right treat to pair with a cup of tea.
Whisk together flour, cocoa, and baking soda. In the bowl of a mixer fitted with the paddle attachment, beat butter with sugars on medium-high speed until pale and fluffy, about 4 minutes. Add salt and vanilla; beat to combine. Add flour mixture; beat on low until just combined. Fold in chocolate. Divide dough in half; shape into two 1 1/2-by-7-inch logs. Wrap each in plastic; refrigerate overnight.
Preheat oven to 350 degrees. Remove logs from refrigerator; let stand until sliceable, about 15 minutes. Using a serrated knife, cut into 1/4-inch-thick slices. (If slices crack, simply press them back together.) Roll edges in granulated sugar; transfer to parchment-lined baking sheets.
Bake, rotating once, until just set around edges and dry to the touch, 14 to 15 minutes. Let cool on sheets 5 minutes, then transfer to a wire rack and let cool completely. Cookies can be stored in an airtight container at room temperature up to 3 days.
Cookie dough can be formed into logs and frozen, wrapped tightly in plastic, up to 3 months.
For a cleaner contrast between the white and dark chocolate, shake chopped chocolate through a sieve to remove any small particles before folding into cookie dough.