Preheat the oven to 180 (350f) and prep an 8-inch baking pan with parchment paper.
In a big mixing bowl, add the room temperature butter, sugar, brown sugar, vanilla extract, cinnamon, nutmeg, and kosher salt, and mix with an electric hand mixer until it's creamy.
Add the eggs, greek yogurt, and vegetable oil. Mix until well combined.
Add the flour, and baking powder, and mix until almost combined.
Add the buttermilk and mix until almost combined.
Add the chopped and peeled apples and mix until just combined.
Pour the batter into a parchment paper-lined baking pan and smooth it out with a spatula.
Make the streusel topping:
Into a medium mixing bowl, add the flour, sugar, cinnamon, and melted butter. Mix with a spoon until the mixture is well combined.
Top the cake with the streusel topping and press the streusel topping onto the cake batter so it sticks and does not fall off while baking.
Bake in a 180c (350f) preheated oven for 45-50 minutes (depending on your oven) or until a toothpick comes out clean. Don’t overbake this cake!
Make a simple glaze:
Into a small mixing bowl, add the powder sugar, milk, and vanilla. Mix with a spoon until the mixture is well combined and until the glaze is smooth and silky.
Top the cake with as much glaze as you would like, and serve warm next to a cup of coffee or hot tea. Enjoy!
Notes
Use room temperature ingredients. It is important that you use room-temperature ingredients like eggs and butter. This step ensures that your mixture mixes into a homogenous mixture and bakes perfectly.
To make this process easy, quick, and clean, use an apple peeler and corer. Here is the one I use all the time.
Measure your flour correctly! When measuring your flour, avoid scooping it with a measuring cup. Instead, stir the flour with a spoon to create as many air pockets as possible, and spoon it into your measuring cup. Finally, level it with a back of a knife.
Don’t overmix the batter, you don’t want to develop the gluten in the flour. This step ensures you have a moist cake and not a dense and dry cake.
Butter your pan with butter and sprinkle flour to prevent sticking. This step helps the cake to bake evenly, and it makes sure that your cake will release easily from the pan.
Make sure that your butter is at room temperature. An hour or two before making this cake, take the butter out of the fridge and let it come to room temperature. The butter should be soft to the touch. If your butter is too cold, it will be harder to mix and there will be lumps of butter in your mixture.
Be patient! I know it’s extremely tempting to eat this cake as soon as it's ready. Allow the cake to cool to room temperature before eating the cake. This step prevents the cake to fall apart.