Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan (metal or glass) and line it with parchment paper, leaving an overhang on two sides to lift the cake out easily.

In a medium bowl, whisk together 1 cup flour, 3/4 cup packed light brown sugar, 1 teaspoon cinnamon, and 1/4 teaspoon fine sea salt. Pour in the melted butter and 1/2 teaspoon vanilla extract. Stir with a fork until the mixture looks like chunky, moist sand and clumps together in small nuggets. Set aside.

In a large bowl, whisk together 2 cups flour, baking powder, baking soda, 1/2 teaspoon fine sea salt, 1 1/2 teaspoons cinnamon, and nutmeg until well combined.

In a separate medium bowl, whisk together the oil, granulated sugar, and 1/4 cup packed light brown sugar until slightly thick and glossy. Whisk in the eggs one at a time, then whisk in the Greek yogurt and 2 teaspoons vanilla extract until smooth.

Pour the wet mixture into the bowl with the dry ingredients. Using a spatula, fold gently just until most of the flour is incorporated. The batter will be thick, similar to muffin batter.

Add the chopped apples and fold them into the batter just until evenly distributed, being careful not to overmix.

Spread about half of the batter into the prepared pan, smoothing it into an even layer. Sprinkle about one-third of the streusel mixture evenly over the batter to create a cinnamon swirl in the center.

Dollop the remaining batter over the streusel layer in small spoonfuls, then gently spread to cover. Crumble the remaining streusel evenly over the top, breaking up any large chunks into smaller pieces.

Bake on the center rack for 38–45 minutes, or until the top is golden brown and a toothpick inserted in the center comes out with just a few moist crumbs. Begin checking around 38 minutes.

Let the cake cool in the pan on a wire rack for at least 20–30 minutes. Use the parchment overhang to lift the cake out onto a cutting board. Slice into 9 large or 12 smaller squares and serve warm or at room temperature.