In a bowl, combine 2/3 cup all-purpose flour, 2/3 cup packed light brown sugar, 1 1/2 teaspoons ground cinnamon, and 1/4 teaspoon salt. Mix until well blended. Remove 1/4 cup of this mixture and place it in a separate small bowl; add the remaining 1/2 teaspoon ground cinnamon to this portion for the filling. Set both the filling and base topping mixture aside.

Add 3 1/2 tablespoons melted and cooled unsalted butter to the reserved base flour/sugar/cinnamon mixture (not the separate filling mixture). Stir until evenly combined and the mixture forms clumps. Cover and set aside for topping the cake later.

Preheat your oven to 350°F (175°C). Line an 8x8-inch square baking pan with parchment paper, leaving a 2-inch overhang on each side; secure the parchment on the pan’s edges with binder clips. In a medium bowl, sift together 1 1/2 cups all-purpose flour, 1 1/2 teaspoons baking powder, and 1/4 teaspoon salt. Add 1/3 cup fine almond flour and whisk to blend evenly.

In a large bowl, combine 1/4 cup softened unsalted butter, 3 tablespoons sunflower oil, 3/4 cup granulated sugar, 1 teaspoon pure vanilla extract, and 1/4 teaspoon pure almond extract. Beat with an electric mixer (or whisk vigorously by hand) for about 2 minutes until the mixture is pale and fluffy. Add the room temperature egg and mix until fully incorporated. Add half of the dry flour mixture from Step 3 and mix on low speed, then alternate adding 2/3 cup whole milk and the remaining dry ingredients in batches, mixing gently until just combined.

Spoon just over half of the cake batter from Step 4 into the prepared pan, spreading it evenly with a small offset spatula. Evenly sprinkle the filling mixture from Step 1 over the batter. Dollop the remaining batter by spoonfuls over the filling and gently spread it out, trying not to disturb the cinnamon sugar layer. Crumble the clumped topping mixture from Step 2 evenly over the batter.

Bake the cake in the preheated oven for 28–32 minutes, or until a skewer inserted into the center comes out clean. Transfer the pan to a wire rack. For a softer crumb topping, I like to immediately cover the cake with a piece of aluminum foil to steam it lightly. Let the cake cool for at least 30 minutes before slicing and serving.