Step 1In a large bowl, sift together the flour and cocoa.
Step 2Using a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, beat together the butter, sugar and salt over medium speed until smooth and creamy, about 3 minutes.
Step 3Reduce the speed to low and blend in the egg white, followed by the vanilla. The dough might curdle, but it will smooth out with mixing and the addition of the flour.
Step 4Still working over low speed, add the flour mixture in 3 or 4 additions, beating only until the ingredients are almost incorporated each time before adding more; scrape down the sides and bottom of the bowl a couple of times as you work, and then continue to mix until the dry ingredients have disappeared into the dough. Wrap the dough tightly and refrigerate until completely chilled.
Step 5Center a rack in the oven and heat it to 350 degrees. Line two baking sheets with parchment paper or silicone baking mats.
Step 6Divide the chilled dough, measuring each portion using a level tablespoon of dough. Roll each portion between your palms to form smooth balls. Return to the baking sheet, spacing each portion about 1½ inches apart. Using the knuckle of your index finger, your thumb or the end of a silicone or wooden spoon, make an indentation in the center of each ball. It’s easiest to do this if you steady the ball of dough with the thumb and index fingers of one hand and make the indent with the other; however, the dough is bound to crack around the edges, and that’s fine. Fill each indentation to the top with jam.
Step 7Bake the sheets, one at a time, until the cookies feel firm to the touch and the jam is bubbling, 16 to 18 minutes. Rotate the sheet halfway through baking for even heating. Transfer the sheet to a rack and allow the cookies to cool for 5 minutes before lifting them onto the rack to cool completely. Repeat with the second sheet.
Step 8If you want to drizzle the cookies with chocolate, melt the chips — you can do this in the microwave or in a bowl set over a pan of simmering water — stirring occasionally until completely melted and smooth. Drizzle the chocolate over the cookies. Refrigerate to set the chocolate, about 30 minutes, then return to room temperature to serve.
Step 9The cookies will keep in a covered container at room temperature for about 4 days, although they will get softer (not a bad thing). Wrapped airtight, they can be frozen for up to 2 months.