Preheat the oven to 350°F (175°C) and line a 9x5 inch loaf pan with parchment paper. Lightly oil it.
Dissolve the instant coffee granules in hot water and let it cool.
In a medium bowl, whisk sugar and eggs until pale and slightly foamy. Add oil, sour cream, vanilla, cooled coffee mixture, and mashed banana. Whisk until smooth.
In another bowl, whisk flour, baking soda, and salt.
Combine dry ingredients into wet and fold gently with a spatula until just combined.
Fold in toasted walnuts. Pour batter into the pan and smooth the top.
Bake for about 60 minutes until a toothpick comes out with moist crumbs.
For the glaze, mix powdered sugar, instant coffee, and hot water until smooth. Drizzle over warm bread.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.