Prep: Preheat oven to 325°F (163°C), middle rack position. Line a 9"x9" (23x23 cm) baking pan with parchment or foil.

Make Filling: Whisk all filling ingredients in a small bowl until fully combined. Set aside.

Make Roux: In a microwave-safe bowl, combine oil and flour. Microwave in 30-second intervals, stirring each time, until mixture bubbles vigorously, about 60-90 seconds total (approx. 250°F/120°C). Allow to cool.

Creaming: In a large bowl, cream sugar and butter with a hand mixer on medium speed until pale and fluffy (about 3 mins). Add eggs one at a time, mixing thoroughly after each. Add vanilla and mix until smooth.

Dry Ingredients: Sift baking powder, baking soda, and salt into the bowl. Mix well to evenly distribute, scraping bowl halfway through.

Flour & Wet Ingredients: Alternate adding flour with sour cream and cooled roux mixture, starting and ending with flour, mixing just until combined.

Assemble: Spread half the batter evenly in the prepared pan. Sprinkle filling evenly on top. Gently spread the remaining batter over the filling.

Make Topping: In the same large bowl (no need to wash), whisk flour, cornstarch, cinnamon, cardamom (if using), salt, and sugar. Rub butter into dry ingredients with your fingers until mixture resembles coarse crumbs. Sprinkle topping evenly over the cake.

Bake: Bake in preheated oven for 60-75 minutes, until golden brown and a skewer inserted in the centre comes out clean.

Cool & Serve: Cool cake in the pan for 15 minutes, then lift it out onto a wire rack. Let it cool further to your preferred serving temperature. Slice and enjoy!