Prepare a 9x9 inch square brownie pan (or cake pan) by buttering and flouring it, lining the bottom with a square of parchment. Set aside
Preheat the oven to 350 ºF (175 ºC).
Streusel
With a fork, stir together all the streusel ingredients in a small bowl. Continue mixing until the streusel clumps a little and all the dry ingredients are moistened. Set aside.
Cake
In a medium bowl, whisk together the flour, baking soda and salt. Set aside for later.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and the light brown sugar. Add the vanilla, followed by the eggs, one at a time, scraping down the bowl as needed between each addition.
With the mixer on low, add the dry ingredients alternately with the sour cream, beginning and ending with the dry ingredients. Give the cake batter a couple stirs with your spatula to be sure all the ingredients have been evenly incorporated and the batter is smooth.
Assembly and baking
Spread the batter evenly in the prepared cake pan.
Top with a layer of the thinly sliced apple. Press them down gently.
Sprinkle the prepared streusel topping over the apples. Press the streusel down gently.
Bake the coffee cake for 40–50 minutes, checking it with a cake tester to be sure it is done.
Let the coffee cake cool completely in the pan on a wire rack before serving.
Notes
I bake with Cortland apples, which are a popular baking apple variety where I live. You may also try Golden Delicious, Gala, Honeycrisp, or even Granny Smith apples. Use an apple variety you are familiar with that doesn't break down when heated.
You can replace the sour cream with crème fraîche or full-fat Greek yogurt (with at least 9 % fat content).
Do not use low-fat sour cream. The cake may be dry if you do.
If you'd like to use more apples, use two: dice one and fold it into the batter before transferring to the cake pan and slice the other as a topping, like in the recipe.