In a large mixing bowl, cream the unsalted butter with the sugar on medium to high speed, about 3-5 minutes. Add the vanilla, instant coffee and lemon zest and mix again. Beat in 2-3 eggs, one at a time. Then add ½ of the amount of flour and mix again, starting on low speed. Now, beat in the rest of the eggs, again, one at a time. Finish with the rest of the flour. Scrape the inside of the bowl to make sure everything is well incorporated. Finally, add the burnt sugar and mix well. Scrape the bowl one more time and mix again to blend everything well.