Preparation Steps

• In a medium bowl, whisk the flour and fine sea salt to combine; set aside.
• In a mixing bowl, beat the butter and sugar with a hand mixer or stand mixer on medium-high until very light and fluffy, 2 to 3 minutes. Beat in the egg yolk and vanilla until smooth, scraping the bowl once.
• Add the dry mixture in two additions on low speed, mixing just until mostly incorporated. Drizzle in the milk and mix only until the dough clumps together and no dry spots remain.
• Flatten the dough into a 1-inch-thick slab, wrap, and chill for 15 minutes. While it chills, heat the oven to 340°F (170°C) and line 2 baking sheets with parchment.
• Scoop 1-tablespoon portions of dough (about 20), roll into smooth balls, and arrange 2 inches apart on the sheets.
• To make the wells, dust a 1/2-teaspoon measuring spoon lightly with flour and press into the center of each ball, giving a small twist to form a neat cavity without cracking the edges.
• Bake one sheet at a time for 12 to 14 minutes, rotating the pan halfway through, until the edges are set and just turning pale golden. Immediately re-press the centers with the measuring spoon to deepen the wells. Cool 5 minutes on the pan, then move cookies to a rack to cool completely.
• For the filling, place the chopped chocolate in a heatproof bowl. Warm the cream in a small saucepan over medium heat until steaming with tiny bubbles at the edges (do not boil). Pour over the chocolate, add espresso powder if using, let sit 1 minute, then stir gently from the center outward until glossy and smooth.
• Transfer ganache to a small piping bag or zip-top bag (snip a tiny corner) and fill the cooled cookies. Let stand at room temperature until set, 30 to 45 minutes. Sprinkle with flaky sea salt before the ganache fully sets, if desired.