Prep the pan and oven. Heat the oven to 350°F (175°C). Grease and line a 9-inch square pan (or a 9-inch springform) with parchment, leaving a little overhang for easy lifting.

Make the streusel. In a bowl, combine 1/2 cup flour, 1/3 cup brown sugar, 1 teaspoon cinnamon, and a pinch of salt. Cut in 4 tablespoons cold butter until clumpy and sandy. Set aside.

Mix the dry ingredients. In a large bowl, whisk 2 cups flour, 1 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon kosher salt, and 1 teaspoon cinnamon.

Cream the butter with the sugar mixture. Add 1/2 cup softened butter to the dry mix and use a hand mixer or sturdy whisk to blend until the mixture looks like damp sand. This reverse-creaming step keeps the crumb tender.

Add wet ingredients. In a measuring cup, whisk 2 large eggs, 1 cup sour cream (or Greek yogurt), and 2 teaspoons vanilla. Pour into the bowl and mix just until smooth. If thick, add 1–2 tablespoons milk. Do not overmix.

Prep the apples. Peel and core 2 medium apples. Dice into small cubes (about 1/2 inch). Toss with 1 teaspoon flour to prevent sinking.

Layer the batter. Spread half the batter into the pan. Sprinkle with half the streusel and half the apples. Add the remaining batter, then top with the rest of the apples and streusel. Gently press the top layer so the apples nestle into the surface.

Bake. Bake 40–50 minutes, until a toothpick in the center comes out clean or with a few moist crumbs. The top should be golden and the edges set.

Cool and glaze (optional). Let cool 20–30 minutes. For a simple glaze, whisk 1/2 cup powdered sugar with 1–2 teaspoons milk and a drop of vanilla until pourable. Drizzle over the warm cake.

Serve. Slice into squares or wedges. Enjoy warm or at room temperature.