If using chocolate bars, chop into bite-sized chunks. Set aside.

In a medium bowl, sift together the cake flour, bread flour, baking soda, baking powder, and salt. Set aside.

In a stand mixer with paddle attachment, cream the butter, light brown sugar, and granulated sugar until very light and fluffy—about 5 minutes.

Beat in the eggs, one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.

Reduce mixer speed to low. Slowly add the dry ingredients and mix just until incorporated (5–10 seconds). Fold in the chocolate chunks, being careful not to break them.

Press plastic wrap against the dough and refrigerate for 24 to 36 hours.

When ready to bake, preheat oven to 350°F. Line baking sheets with parchment paper. Scoop 6 mounds of dough (about 3½ oz each) per tray, spacing well apart. Turn any exposed chocolate pieces horizontally for prettier cookies.

Sprinkle with sea salt flakes, if using. Bake for 16–18 minutes, until golden but soft in the center.

Cool on the tray for 10 minutes, then transfer to a wire rack.

Melt 10 oz of bittersweet chocolate. Once cookies are fully cooled, spoon melted chocolate onto the bottom of each cookie and spread evenly.

Place cookies chocolate side up on a parchment-lined sheet and chill until chocolate sets.