Preheat the oven to 180 C / Gas 4. Grease and flour a ring cake tin.
To make streusel: In a medium bowl, mix brown sugar, 100g flour and cinnamon. Rub in the butter with your fingers or a fork, until crumbly. Stir in walnuts.
In a medium bowl, stir together 400g flour, baking powder, and bicarbonate of soda. In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, mixing well after each. Then stir in the vanilla and yogurt. Gently stir in the flour mixture just until blended.
Pour 1/2 of the batter into the cake tin, sprinkle with 1/4 of the streusel and layer with apples. Sprinkle with 1/2 the remaining streusel. Pour in the remaining batter, and top with the remaining 1/4 streusel. Lightly pat the top layer of streusel so it sticks to the cake batter.
Bake 50 to 60 minutes in the preheated oven, or until a knife inserted in the centre comes out clean. Cool in the tin for 15 minutes, then carefully turn out onto a wire cooling rack. Leave to cool completely before serving.
Exactly the 'coffee cake' recipe I was looking for - a hidden layer sandwiched in the middle and no heavy sweet icing - the perfect cake to have with a cup of coffee. The recipe does take some prep but it makes a lot of cake - I divided the batter and made two deep round cakes and they still made very generous portions. Cooked my two cakes together for around 1hr10 - when the skewer came out clean. Delicious! - 10 Oct 2016