Heat slightly the water in a pot, add the instant coffee, vanilla extract and sugar. Stir with a whisk until brown sugar is melted completely and let cool down.
Sift flour in a bowl along with starch and baking powder.
Pour the oil and other liquid ingredients in the bowl little by little while stirring with a whisk to obtain a smooth and homogeneous batter.
Pour the cake batter into a 9" cake pan lined with parchment paper, sprinkle with chopped walnuts and bake at 320°F 45 minutes.
Note
The coffee and walnut cake keeps good and moist up to 5 days at room temperature, under a cake glass dome or airtight container.