Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper or silicone baking mats.
Beat the softened butter and granulated sugar together until smooth and creamy, about 2 to 3 minutes.
Mix in the vanilla extract until fully incorporated.
In a separate bowl, whisk together the flour, espresso powder, and salt.
Gradually add the dry ingredients to the butter mixture. Mix on low speed just until a soft dough forms. Do not overmix.
Shape the dough into a flat disk, wrap it tightly, and refrigerate for about 30 minutes.
Roll the chilled dough to approximately 1/4 inch thick on a lightly floured surface. Cut into your preferred shapes and arrange the cookies on the prepared baking sheets.
Bake for 10 to 13 minutes, or until the cookies are set with lightly golden edges. Avoid overbaking.
Let the cookies cool on the baking sheet for about 5 minutes, then transfer them to a wire rack. Cool completely.
Melt the chopped chocolate gently in a microwave-safe bowl in short intervals, stirring between each interval. You can also use a double boiler. Stir in the optional oil if desired.
Dip one end or half of each cooled cookie into the melted chocolate. Allow excess chocolate to drip back into the bowl.
Place the dipped cookies on parchment paper. Add flaky sea salt, chopped nuts, espresso powder, or another topping while the chocolate is still soft.
Let the chocolate set completely at room temperature or chill the cookies for 10 to 15 minutes.
