Start by making the crumble – while the first quantity of browned butter is still liquid (but not hot), mix in the first quantity of sugar and brown sugar. Add the first quantity of sieved flour, salt and cinnamon only until the mixture comes together, don’t - over mix. Keep aside.
In a large bowl using a stand mixer or hand held mixer, cream together the second quantity of softened browned butter and sugar together until very fluffy. Slowly add the eggs.
In a medium sized bowl, sieve together the second quantity of flour, baking soda, baking powder, cinnamon and salt. Fold into the creamed butter mixture. Finish with folding in the Greek yoghurt, vanilla and diced apple.
Put in a 25x20cm rectangle baking tin lined with baking paper. Top with the crumble and bake in a preheated oven at 170°C for 30 minutes or until skewer comes out clean.
When the cake is cooled, mix together the final 3 ingredients and drizzle on top.