With an electric mixer on medium-highspeed, cream butter and sugar until pale andfluffy. Beat in whole eggs and salt. Add vanilla,milk, flour, and cocoa; mix until incorporated.
Turn out dough onto a piece of plasticwrap, and shape into a rectangle; wrap. Chilluntil firm, at least 1 hour, or up to 1 day.
On a generously floured 12-by-26-inch sheetof parchment paper, roll out chocolate doughto an 11-by-24-inch rectangle, dusting with flouras needed and occasionally lifting dough withan offset spatula to prevent sticking. Chill again.
On another generously floured piece ofparchment, roll out almond dough to a 9 1/2-by-24-inch rectangle, as described above.
Lightly beat egg white; brush over chocolatedough. Gently stack almond dough ontop, aligning flush on one long side (there willbe a 1 1/2-inch space on the other long side).Cut stack crosswise into thirds to make three8-by-11-inch rectangles. Starting with theshort side where the edges are flush, rolldoughs together into a log, gently liftingparchment as you go. Lightly brush edge withegg white; seal seam by pinching and pressinggently. Wrap log in parchment or waxedpaper. Repeat with remaining two stackedrectangles. Chill in freezer until firm, about 1 1/2 hours, or up to 1 month.
Preheat oven to 350 degrees. Line baking sheetswith parchment paper. Let dough stand atroom temperature 10 minutes. Using a serratedknife, cut log into 1/4-inch-thick rounds, rotating log after each cut to keep it round.Transfer rounds to sheets. Bake until setbut not taking on any color, 18 to 20 minutes.Transfer cookies (still on parchment) towire racks to cool completely. Cookies canbe stored up to 5 days at room temperaturein airtight containers.