Preheat oven to 340F (170C).In a small bowl whisk instant coffee and hot water. Set aside.In a large bowl, cream together butter and icing sugar. Add egg yolk and stir until combined. Add coffee and whisk until combined. Sift in flour, cornstarch, cocoa powder and salt. Stir until incorporated and dough is formed. Refrigerate for 20-30 minutes.Scoop out 1 teaspoon of the dough and roll then using your palms, shape them gently into an oval. Arrange then on a baking tray lined with a parchment paper. Then, using a toothpick or a back of a knife press it gently along the diameter of the oval.Freeze for 10 minutes or refrigerate for 30 minutes.Bake for 12 minutes. Allow to cool on a wire rack before serving.Notes:Is cornstarch and corn flour the same?I am often asked by European visitors if cornstarch and corn flour are the same thing. Yes, cornstarch and corn flour are the same thing. The term “Cornstrach” is used in north America, while “corn flour” is used in Europe and Asia. But in America the term corn flour refers to finely ground cornmeal, so it's important not to get confused.What can we use instead of cornstarch?Cornflour gives cookies a “melt in your mouth” effect. If you do not have cornstarch on hand, you can use the same amount of all-purpose flour or cakr flour.What kind of coffee can we use?To make coffee bean cookies you can use instant coffee or if you don’t have instant coffee, you can use 2 tablespoons of brewed coffee or same amount of espresso.How long can we store the cookies?The cookies can be stored in an airtight container, refrigerated, for up to 5 days.Can we make coffee bean cookies ahead of time?These cookies can be made ahead of time. You can prepare the dough, shape into coffee beans as described in the recipe and freeze in for up to 2 months. Then, defrost in the refrigerator overnight and bake as described in the recipe.More Cookie Recipes:Melt in Your Mouth Butter CookiesFrench MacaronsFudgy Brownie Cookies3-Ingredient Coconut CookiesChocolate Crinkle Cookies