Brush the sides of the pan generously with oil. Add 1/4 cup flour to it. Cover with an aluminium foil and shake well to coat the sides of the pan. Set aside.
Make Walnut Struesel
Take chopped walnuts, cinnamon and brown sugar in a bowl. Mix with a rubber spatula to coat the walnuts well. Set aside.
Coffee Cake Batter
In a large bowl, whisk the yogurt and coffee powder until it is light, fluffly and smooth about 3 mins or so.
Add the rest of the wet ingredients - oil, sugar and vanilla. Whisk well.
Place a sieve on top and directly sieve the dry ingredients - flour, cornflour, baking powder, baking soda and salt into it.
Fold together using a spatula to make a cake batter like consistency.
Pour half of the batter on to the prepared cake pan. Layer it with walnut crumble and top it off with the remaining cake batter.
Bake this in a preheated oven of 180 degree celsius for 28 to 38 mins or until a skewer inserted in the centre comes out clean.
Let it sit in the pan itself for another 5 to 8 mins and then invert on a wire rack to cool completely.
Serve plain with a cup of coffee and add some lemon icing on top before serving.
Notes
Prepare your pan properly: If you're using a bundt pan, make sure to grease it thoroughly with oil or cooking spray. Dusting it with flour or cocoa powder can also help prevent sticking. Do not use butter for greasing as the milk solids in the butter actually causes the cake to stick to the pan.
Use parchment paper: If you're baking in a rectangular or square pan, you can line the bottom of the pan with parchment paper to prevent sticking.
Use the right type of flour: For the best texture, use cake flour instead of all-purpose flour. Cake flour has a lower protein content, which helps to create a tender crumb. And that is the reason why we are using cornflour in this recipe. Plain flour + Cornflour = Homemade Cake Flour.
Don't overmix the batter: Overmixing can cause your cake to be tough and dense. Mix your ingredients until just combined, and then stop.
Add some flavor: Coffee cakes are often flavored with cinnamon, nutmeg, or other spices. Don't be afraid to experiment with different flavors to find your favorite combination.
Make a crumb topping: A crumb topping adds texture and flavor to your cake. Mix flour, sugar, and butter together until crumbly, and then sprinkle it over the top of the cake or sandwich them like in this recipe before baking. Don't be afraid to experiment with different flavors in your streusel or crumb topping. You can add chopped nuts, dried fruit, or even chocolate chips for an extra pop of flavor.
Test for doneness: Use a toothpick or cake tester to check if your cake is done. Insert it into the center of the cake, and if it comes out clean, your cake is ready.
Let it cool: Allow your cake to cool in the pan for 5-8 minutes before removing it to a wire rack to cool completely. This will help prevent it from breaking apart.
Add a glaze or icing: A simple glaze or icing can add a nice finishing touch to your coffee cake. You can make a glaze by mixing powdered sugar and milk, or make icing by mixing powdered sugar, milk, and vanilla extract or powdered sugar and lemon. Drizzle it over the cooled cake for a sweet and pretty finish.