Preheat the oven to 180°C / 350°F and place a rack in the middle position. Line a baking sheet with parchment paper or a silicone baking mat.

Place the 4 tablespoons (55g) cubed unsalted butter and 4 ounces (115g) finely chopped unsweetened chocolate in a large heatproof bowl. Melt together over a double boiler, stirring until smooth, or melt in the microwave in short bursts (20–30 seconds), stirring between bursts, until just melted. Do not overheat.

Off the heat, stir in 1 cup (200g) granulated sugar, 1/2 teaspoon salt, and 1 teaspoon vanilla extract until combined.

Let the chocolate mixture cool briefly (about 1 minute), then add the 2 large eggs one at a time, mixing thoroughly after each addition until fully incorporated.

Add 1 cup (140g) all-purpose flour and 1 teaspoon baking powder to the bowl. Stir gently just until there are no visible streaks of flour; do not overmix.

Divide the 3/4 cup (85g) chopped walnuts into two portions: stir 1/2 cup into the batter and reserve 1/4 cup for topping.

Use a small ice cream scoop or spoon to portion the batter into 12 even mounds on the prepared baking sheet, spacing them evenly.

Sprinkle the reserved 1/4 cup chopped walnuts over the tops of the cookie mounds.

Bake on the middle rack until the cookies are puffed, cracked on top, and just set—about 8 minutes. Avoid overbaking.

Remove the baking sheet from the oven and let the cookies cool on the sheet for 5 minutes, then transfer them to a wire cooling rack to cool completely.