Preheat the oven to 180℃ and line your bundt tin with baking spray.
In a stand mixer fitted with the paddle attachment start by beating your butter and sugar until pale and light. Add the eggs one at a time beating well between each interval. Add the vanilla and beat again.
In a bowl add lemon juice into the yogurt and stir around. We start by adding the yogurt and the flour into the batter in 3 parts. Beating well between each interval.
Scrape down the sides and give the batter one more mix before setting side.
To make the streusel topping we mix the flour, oats, brown sugar and cinnamon powder together. Add the butter and rub together until you have clumps and damp sand like texture. Add most of the walnuts and set aside.
We add half the cake batter into the tin and evenly smoothen it out. We add half the streusel mixture on top and layer with the rest of the cake batter and top with the remaining streusel mixture.
Sprinkle the leftover walnuts and Demerara sugar before baking at 180℃ for 50-55 mins until a tooth pick inserted into the center comes out clean. Let the cake cool for 1-2 hours before removing and icing.
While the cake cools we make the icing by adding a tbsp lemon juice into the icing sugar until it forms dripping consistency. Keep adding a little at a time and whisk well, this takes time but be patient.
Drizzle the icing on top of the cake once it has cooled. Let it set for a minute before slicing and serving.