Prep the pan: Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Toast the walnuts: Spread walnuts on a baking sheet and toast at 350°F (175°C) for 7–9 minutes, until fragrant. Cool, then chop into bite-size pieces.

Set up a gentle melt: In a medium, heavy-bottomed saucepan, combine chocolate chips, sweetened condensed milk, and butter. Add espresso powder if using.

Melt low and slow: Warm over low heat, stirring constantly with a spatula until smooth and glossy. Do not let it simmer or scorch.

Add flavor: Remove from heat. Stir in vanilla and fine sea salt until fully combined.

Fold in walnuts: Add chopped, cooled walnuts and fold gently so they’re evenly distributed without crushing them.

Fill the pan: Scrape the mixture into the lined pan. Smooth the top with an offset spatula. If you like, sprinkle a pinch of flaky sea salt over the surface.

Chill to set: Refrigerate for 2–3 hours, or until firm enough to slice cleanly.

Slice and serve: Lift the fudge out by the parchment. Use a warm, sharp knife (run under hot water and dry) to cut into 1-inch squares. Wipe the knife between cuts for neat edges.