Prepare the filling: In a bowl, mix together the flour, brown sugar, 1½ teaspoons of cinnamon, and salt until fully combined. Scoop out ¼ cup of this mixture into a separate small bowl and stir in the remaining ½ teaspoon of cinnamon—this reserved portion will be used as the filling.

Prepare the topping: To the remaining flour-sugar mixture, pour in the melted butter and stir until the mixture is fully moistened and forms soft clumps. Cover the bowl and set it aside while you make the batter.

Preheat your oven to 350°F (175°C). Line an 8x8-inch square baking pan with parchment paper, leaving a 2-inch overhang on two sides to make lifting easier after baking. Secure the overhang with ¾-inch binder clips to keep the paper in place.

Mix the dry ingredients: Sift the all-purpose flour, baking powder, and salt into a medium bowl. Add the almond flour and whisk everything together until evenly combined.

Make the cake batter: In a large mixing bowl, combine the softened butter, sunflower oil, granulated sugar, vanilla extract, and almond extract. Beat with an electric hand mixer for about 2 minutes until the mixture becomes pale and fluffy. Alternatively, use a whisk and beat vigorously until the same consistency is reached. Add the egg and beat until fully incorporated. Mix in half of the dry ingredients on low speed, then alternate adding the milk and the remaining flour mixture while continuing to mix on low, just until the batter is smooth and combined—do not overmix.

Assemble the cake: Spoon a bit more than half of the batter into the prepared pan and spread it into an even layer using a small offset spatula. Sprinkle the filling mixture evenly over the batter. Dollop the remaining batter over the filling in spoonfuls, spacing them out across the surface. Gently spread the dollops into one another using the offset spatula, being careful not to disturb the layer of cinnamon-sugar underneath. Once the batter blobs start to connect, continue to smooth the surface and push the batter gently out to the edges. Finally, take handfuls of the prepared topping mixture and crumble it evenly across the surface.

Bake for 28–32 minutes, or until a skewer inserted into the center comes out clean. Transfer the pan to a wire rack to cool. If you prefer a soft crumb topping rather than a crunchy one, loosely cover the hot cake with a piece of aluminum foil immediately after baking to allow it to steam slightly. Let the cake cool for at least 30 minutes before slicing and serving.