In a small bowl, combine the sugars, salt, coconut and flour. Add the melted butter and press the mix together. Set aside until ready to use.
Make the cake:
Preheat the oven to 350 degrees F. Prepare a 9 by 9-inch baking dish with parchment paper.
In a mixing bowl, combine the flour, baking powder and salt.
In a separate bowl, cream together softened butter and sugar. Beat in the eggs, one at a time. Add the mashed purple yam, and stir until well combined. Beat in half of the flour mixture, then the coconut milk. Gradually add in the remaining flour mix until well combined.
Pour the batter into the prepared pan. Break the topping into big chunks and sprinkle over the batter. Bake for 45 to 50 minutes or until a toothpick comes out clean. Cool on a wire rack.
Make the glaze:
Whisk glaze ingredients together until smooth. Drizzle glaze over coffee cake before serving.