In a stand mixer fitted with the paddle attachment, cream the softened butter and light brown sugar on medium speed for 3 minutes until light and aerated.

Add the vanilla extract and instant espresso powder; beat for 1 minute until the powder is fully dissolved into the fat.

Whisk the all-purpose flour and salt in a separate container, then add to the mixer on low speed in two batches, mixing until just combined.

Fold in the toasted chopped walnuts by hand using a silicone spatula to ensure even distribution without overworking the gluten.

Transfer the dough to a sheet of plastic wrap and shape into a log approximately 2 inches in diameter. Wrap tightly and refrigerate for 60 minutes to firm.

Preheat the oven to 350°F (175°C) and line two heavy-gauge baking sheets with silicone mats or parchment paper.

Slice the chilled dough log into 1/2-inch (1.25 cm) thick rounds and arrange them 2 inches apart on the prepared sheets.

Bake for 12 to 15 minutes, or until the edges are set and very lightly golden; do not overbake as shortbread should remain pale.

Allow the cookies to rest on the baking sheet for 5 minutes before transferring to a wire cooling rack to reach room temperature.

Prepare the ganache by placing chopped dark chocolate in a small heat-proof bowl. Bring heavy cream to a simmer in a saucepan, then pour over the chocolate.

Let the chocolate and cream sit undisturbed for 5 minutes, then whisk from the center outward until a smooth, glossy emulsion forms.

Dip one half of each cooled cookie into the ganache, allowing excess to drip off, and place on wax paper to set completely before serving.