In a medium saucepan over medium-high heat, melt the honey and butter. Transfer the mixture to the bowl of a stand mixer and beat on medium-high until cooled to room temperature, about 3 minutes. Add the eggs, one at a time, beating between each.
A third at a time, beat in the dry ingredients, scraping down the sides of the bowl between additions. Spoon the mixture into the prepared pan and bake for 25 minutes or until a toothpick inserted at the center of the cake comes out clean. Allow to cool in the pan for 15 minutes, then turn out onto a wire rack to cool completely.
Meanwhile, in a small bowl, whisk the water, instant coffee and vanilla until the coffee granules are completely dissolved. Whisk in the powdered sugar until smooth. Drizzle the mixture over the cake and serve topped with fresh mango.