Make the streusel topping: Mix together flour, 3/4 cup brown sugar, 1 teaspoon cinnamon, and 1 teaspoon salt. Cut in butter using a pastry cutter or rub in with your fingers until small to medium clumps form. Mix in 1/2 cup walnuts. Refrigerate until ready to use to prevent the butter from reaching room temperature.

Make the streusel center: Mix together remaining 1/4 cup brown sugar, 1/4 teaspoon cinnamon, and 1 cup walnuts.

Preheat oven to 350F/176C degrees.

Make the cake: Butter a 9-inch tube pan with a removable bottom. Sift flour, baking powder, baking soda, and 1/2 teaspoon salt into a bowl.

Beat butter and granulated sugar with a mixer on medium speed until pale and fluffy, about 2 minutes. Beat in eggs, 1 at a time, then vanilla. Beat in flour mixture in 3 additions, alternating with the Greek yoghurt, beginning and ending with flour. Continue to beat until well combined.

Spoon half the batter into pan. Sprinkle streusel center mixture evenly over batter. Top with remaining batter, and spread evenly using an offset spatula. Sprinkle streusel topping mixture evenly over batter.

Bake until cake is golden brown and a toothpick inserted into the center comes out clean, about 55 minutes. Transfer pan to a wire rack, and let cool completely. Remove cake from pan, and transfer to parchment.

Make the glaze: Mix together confectioners' sugar and milk. Drizzle over cake, and let drip down sides. Let set for 5 minutes before serving.