Preheat the oven to 180℃. Line a 9" square cake tin with baking parchment and lightly grease.
To make the topping, blitz the dates and walnuts in a food processor till they resemble small chunks. Stir in the salt, cinnamon powder and coconut oil and set aside.
Sift together the salt, soda bicarb and baking powder in a bowl.
In another bowl, cream the butter until soft, add the sugar and beat until the mixture is pale and fluffy.
Beat in the eggs one at a time.
Fold in the flour in three batches alternating with the yoghurt until fully incorporated.
pour half the batter into the prepared cake tin and level with the back of the spoon. Sprinkle half the crumble mix over the batter.
Pour the remaining cake batter over the crumble and top with the remaining crumble mix.
Bake for 35-40 min or until a skewer inserted into the cake comes out clean. If the cake is still under after 40 mins, bake for a further 10 mins.
Rest the cake in the tin for 5 min before turning out onto a wire rack to cool. Once cooled slice into squares.