In a largish bowl, with an electric mixer, beat the sugar and butter at medium speed for about 4 minutes). Add egg whites and beat well. Beat in lemon zest and vanilla extract.
Lightly whisk together the flour, salt, baking powder and baking soda. Add flour mixture to and the buttermilk, alternately, to the butter-sugar mixture beginning and ending with flour mixture. Using a hand whisk, lightly mix after each addition till well blended.
Add the mango pieces and fold in lightly so it is evenly dispersed in the batter. Scrape the batter out into a greased and floured bundt pan (or loaf pan - 8u201d by 4u201d/ 8u201d cake tin).
Bake at 180C (350F) for about 35 to 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes, then remove from the pan and cool on a rack.
Mix all the ingredients for the sugar glaze, adding the liquid in bits till you have a glaze of a thick pouring consistency. Pour the sugar glaze on a slightly warm cake and let it cool completely.