First make the cake. Start by heating the ghee/ butter in a pan. Add the broken cashewnuts and pan fry them, stirring frequently, till they turn golden. Remove the cashewnuts and keep aside. Add any remaining bit of ghee to the butter and sugar while making the cake.
You can do the mixing in a food processor, but I used my hand held mixer. Put the butter, sugar and vanilla extract in a large bowl. Beat, on low speed, till everything is blended. Add the egg, milk, flours, salt and baking powder and beat again on low speed till well blended.
Scrape the batter into a greased and floured 8u201d (or 9u201d) cake tin and lightly smooth the top. Uniformly scatter the cubed mango pieces and then the cashewnuts over the top.
Bake the cake at 190C (375F) for 15 minutes. Then take it out and sprinkle the topping evenly on top.
In the meanwhile, make the streusel/ crumb topping.
Powder the oats to a coarse powder. Put this and all the ingredients for the topping, except the butter, in a bowl and stir together. Cut the cold butter into small pieces and, using your fingers, rub them into the flour till the mixture forms small clumps.
After 15 minutes of baking, take the cake out and sprinkle the topping evenly on the cake. Put the cake back in the oven and continue to bake at 190C (375F) for about 30 to 40 minutes, till the crumb topping/ streusel is brown and the cake is cooked.
Cool the cake completely and then slice. Serve with coffee or tea, or perhaps for dessert with sorbet or ice-cream.