Prepare the Streusel Topping: In a medium bowl, toss together the flour, brown sugar, and cinnamon. Add the softened butter. Now, here's my favorite part – use your fingers or a pastry blender to mix it all up until it forms coarse, crumbly pieces. It should look like delicious little pebbles! Set this aside while you make the batter.
Make the Coffee Cake Batter: First things first, preheat your oven to 350°F (175°C). Grease a 9-inch baking pan – I usually use butter or cooking spray. In a medium bowl, give a good whisk to your flour, baking powder, baking soda, and salt. This just ensures everything is nicely combined. In a separate, larger bowl, cream together the softened butter, granulated sugar, and brown sugar. Beat them with an electric mixer until the mixture is light, fluffy, and looks a bit pale. Then, beat in the egg and vanilla until well combined. Now, it’s time to add the dry ingredients and the buttermilk. Add about a third of the flour mixture to the butter mixture and mix on low speed. Then, add half of the buttermilk and mix again. Repeat this process, alternating between the flour mixture and the buttermilk, starting and ending with the flour mixture. Mix only until everything is just combined – overmixing can make the cake tough.
Prepare the Apple Filling: In a small bowl, mix together the brown sugar, flour, and cinnamon. Add your diced apples and toss them gently until they're all nicely coated in that sweet, spicy mixture. This little step makes such a difference in the flavor and texture of the apple layer!
Assemble the Cake: Grab that greased baking pan. Spread about half of your lovely coffee cake batter evenly into the bottom. Now, carefully scatter the apple filling over the batter. Try to get an even layer so you get apples in every bite! Then, dollop the remaining batter over the apples, spreading it gently to cover them as much as possible. Finally, take your prepared streusel topping and sprinkle it evenly all over the top of the cake. Get every corner!
Bake: Pop that beautiful creation into your preheated oven. Bake for about 35-40 minutes. You'll know it's ready when a toothpick inserted into the center comes out clean, and the top is a lovely golden brown. Let the cake cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving. This allows it to set up properly.