Preheat and prep: Heat your oven to 350°F (175°C). Grease an 8x8-inch square pan and line it with parchment, leaving overhang for easy lifting.

Make the cinnamon swirl and crumble: In a small bowl, mix 1/2 cup brown sugar with 2–3 teaspoons ground cinnamon and a pinch of salt. For the crumble, combine 1/3 cup gluten-free flour, 1/3 cup brown sugar, 1 teaspoon cinnamon, and a pinch of salt. Stir in 3 tablespoons melted butter until clusters form. Set both aside.

Whisk the dry ingredients: In a large bowl, whisk 1 1/2 cups gluten-free all-purpose flour with 2 tablespoons cornstarch, 1 1/2 teaspoons baking powder, 1/4 teaspoon baking soda, and 1/2 teaspoon fine sea salt.

Mix the wet ingredients: In a separate bowl or large measuring cup, whisk 2/3 cup granulated sugar, 1/4 cup neutral oil, 2 large eggs, 1 teaspoon vanilla, 1/2 cup sour cream, and 1/3 cup milk until smooth.

Combine gently: Pour the wet mixture into the dry ingredients. Stir with a spatula just until no dry streaks remain. The batter should be thick but spreadable. Avoid overmixing.

Layer the cake: Spread half the batter into the pan. Sprinkle half the cinnamon-sugar mixture over it. Dollop the remaining batter on top and gently spread. Sprinkle the remaining cinnamon sugar, then scatter the crumble over everything.

Bake: Bake for 32–40 minutes, until the top is golden and a toothpick inserted in the center comes out mostly clean with a few moist crumbs. If the top browns too quickly, tent loosely with foil for the last 10 minutes.

Cool and glaze (optional): Let the cake cool in the pan for at least 20 minutes. If you’d like, whisk 1/2 cup powdered sugar with 1–2 tablespoons milk and a drop of vanilla, then drizzle over the slightly warm cake.

Slice and serve: Lift the cake out using the parchment and cut into squares. Serve warm or at room temperature with coffee or tea.