Preheat the oven to 350 degrees. Butter and flour a 4½-by-8½-inch loaf pan. In a small bowl, stir together the rum or brandy and the instant espresso until the espresso dissolves.

In a mixer fitted with the whisk attachment, beat the egg whites with a pinch of salt until they are frothy. Add ¼ cup of the sugar, and beat at high speed until the egg whites form firm peaks, about 1 to 2 minutes. Set aside.

In a clean bowl in the mixer fitted with the paddle attachment, cream the butter and remaining ¾ cup sugar at medium-high speed until light and fluffy, about 2 minutes. Add the yolks, beating well between additions. Beat in the vanilla and the espresso-rum mixture. Sift the flour directly over the bowl, and mix just to combine.

 Remove the bowl from the mixer, and stir in about a quarter of the egg whites to lighten the batter. Gently fold in the remaining egg whites. Fold in the walnuts. Scrape the batter into the prepared pan, and bake until the cake is golden and a tester comes out clean, about 50 minutes.

Cool it on a rack before unmolding and slicing.