Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. This prevents sticking and makes cleanup easier.
Whisk together the flour, espresso powder, and salt in a medium bowl until well combined and no espresso clumps remain. The espresso needs to be evenly distributed throughout the flour.
Beat the butter and sugar together in a large bowl using an electric mixer on medium speed until light and fluffy, about 3 minutes. Don’t rush this step – proper creaming creates the perfect cookie texture.
Add the vanilla and egg yolks to the butter mixture and beat until fully incorporated and smooth. The mixture should look creamy and slightly lighter in color.
Gradually add the flour mixture to the wet ingredients on low speed, mixing just until combined. Overmixing will develop too much gluten and make tough cookies.
Roll the dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets. You should get approximately 24 cookies.
Make an indentation in the center of each ball using your thumb or a measuring spoon. If the dough cracks, gently press it back together – slightly damp fingers can help with this.
Bake for 12-15 minutes until the edges are set but not browned. The cookies should remain pale to maintain their espresso color.
If the indentations have puffed up, gently press them down again while the cookies are still warm. Work quickly but carefully.
Allow the cookies to cool completely on the baking sheets. Moving them too soon may cause them to break.
Fill each indentation with caramel sauce when cookies are completely cool. Warm the caramel slightly if it’s too thick to pour easily.
If using, drizzle melted chocolate over the cookies and sprinkle with a tiny pinch of flaky sea salt for that perfect sweet-salty finish.