Toast the Walnuts: Preheat your oven to 175°C (350°F). Spread the walnuts in a single layer on a parchment-lined baking sheet. Toast for 6–8 minutes, stirring halfway through, until fragrant and lightly golden. Let cool completely, then chop roughly. This step intensifies their flavour dramatically - do not skip it.
Prepare the Streusel: In a medium bowl, combine the toasted chopped walnuts, brown sugar, flour, cinnamon, nutmeg, and cloves (if using). Add the cold cubed butter and work it in with a fork or your fingertips until the mixture resembles coarse, clumpy crumbs. Set aside.
Make the Cream Cheese Filling (Optional): In a small bowl, beat together the softened cream cheese, sugar, egg yolk, and vanilla extract until smooth and creamy. Set aside. This filling adds a rich, tangy layer that pairs beautifully with the cinnamon and walnut.
Make the Cake Batter: Grease and flour your Bundt pan (or 9x13 baking pan) thoroughly. In a large bowl, cream together the softened butter and granulated sugar with an electric mixer on medium-high speed for 3–4 minutes until pale and fluffy. Add the eggs one at a time, beating well after each addition. Mix in the sour cream and vanilla extract on low speed until combined.
Add Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Add the dry ingredients to the wet mixture in two additions, alternating with the milk, beginning and ending with the flour. Mix on low speed just until combined - do not overmix.
Assemble the Cake: Spread half of the cake batter into the prepared pan. If using the cream cheese filling, drop spoonfuls evenly over the batter. Sprinkle half of the cinnamon walnut streusel over the top. Add the remaining batter carefully on top and smooth with a spatula. Finish with the remaining streusel, pressing lightly so it adheres.
Bake: Bake at 175°C (350°F) for 55–65 minutes (45–55 minutes for a 9x13 pan), or until a toothpick inserted in the centre comes out clean. If the top is browning too quickly, tent loosely with aluminium foil for the last 10–15 minutes.
Cool the Cake: Let the cake cool in the pan on a wire rack for 30 minutes. If using a Bundt pan, carefully invert onto a serving plate. For a 9x13 pan, simply let it cool in the pan.
Make the Vanilla Glaze: In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth and pourable. Adjust the milk - add more for a thinner drizzle, less for a thicker glaze.
Glaze and Serve: Drizzle the vanilla glaze generously over the cooled cake. Let the glaze set for 5–10 minutes before slicing. Serve at room temperature with coffee, tea, or your favorite warm beverage.
