1. Place ¼ cup milk chocolate chips, ¾ cup semisweet chocolate chips and butter in heatproof bowl. Place over a pan of almost simmering water and stir constantly with rubber spatula until chocolate and butter have melted. (It may look like the chocolate is about to seize, but don’t worry—the vegetable oil in the next step will smooth it out.) If chocolate melts before butter, you can take bowl off of pot and stir until butter is melted. (You can also do this step in the microwave in 30-second bursts, stirring after each one until mixture is almost fully melted. Stir to melt completely; do not overheat.)

2. Using a whisk, stir vegetable oil, espresso powder (if using) and vanilla into chocolate mixture until smooth. Set aside.

3. In medium bowl, mix together sugar, eggs and egg white until well combined.

4. In small bowl, sift together brown sugar, salt, potato starch and cocoa powder. Add to bowl with sugar and eggs. Stir to combine.

5. Add melted chocolate mixture and then fold in remaining ½ cup semisweet chocolate chips and ¼ cup milk chocolate chips. (Dough will look very liquid-y, like brownie batter.) Refrigerate dough for one hour.

6. After one hour, preheat oven to 350 degrees. Line two baking sheets with parchment paper. Measure 1-tablespoon portions onto each sheet, leaving about 3 inches between each portion (do not skimp on the space between each portion, as the dough will run together). Bake side by side if your oven can fit it, or else on the upper middle and lower middle racks for 12-13 minutes, switching and rotating baking sheets halfway through baking. Do not over-bake. The cookies are done when their edges are set, but middles are still soft. Transfer on parchment paper to cooling rack and let cool for 10 minutes.

Reprinted with permission from JewishBoston.com.