📖 Instructions

1. Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan. This step is crucial; trust me, you don't want your beautiful cake sticking to the pan!
2. In a large bowl, cream together the butter and sugar until light and fluffy. It should look like a cloud – a sweet, buttery cloud. This usually takes about 3-5 minutes with an electric mixer. Don't rush this step; it's important for the cake's texture.
3. Beat in the eggs one at a time, then stir in the vanilla extract. Make sure each egg is fully incorporated before adding the next. I once added all the eggs at once and the batter was...weird. Learn from my mistakes!
4. In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. This ensures that the baking powder and soda are evenly distributed, resulting in a cake that rises evenly. Ever had a cake with a weird bump in the middle? Yeah, probably wasn't mixed well enough.
5. Gradually add the dry ingredients to the wet ingredients, alternating with the yogurt, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix – overmixing develops the gluten in the flour, leading to a tough cake. We want tender, not tough!
6. Gently fold in the diced apples and chopped walnuts. Distribute them evenly throughout the batter. Imagine each bite bursting with apple and walnut goodness. Sounds amazing, right?
7. Pour the batter into the prepared baking pan and spread evenly. Now, for the streusel topping!
8. In a medium bowl, combine the flour, brown sugar, cinnamon, and cold butter. Use a pastry blender or your fingers to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. This is where the magic happens! The cold butter creates little pockets of air, resulting in a crumbly, delicious topping.
9. Sprinkle the streusel topping evenly over the cake batter. Don't be shy! More streusel is always better.
10. Bake in the preheated oven for 35-40 minutes, or until a wooden skewer inserted into the center comes out clean. The cake should be golden brown and spring back lightly when touched. If the top starts to brown too quickly, you can loosely cover it with foil.
11. Let the cake cool in the pan for 10-15 minutes before slicing and serving. This prevents the cake from crumbling when you cut into it. Patience is a virtue, my friends!
12. Serve warm, with a dollop of whipped cream or a scoop of vanilla ice cream, if desired. Or just eat it as is! It's delicious either way. A cup of hot coffee or tea is the perfect complement. Enjoy! I know I will.