Step 1: Prepare the Crumb Topping: In a medium bowl, combine 1 cup all-purpose flour, 1/2 cup granulated sugar, 1 teaspoon ground cardamom, and 1/4 teaspoon salt. Add the cold, cubed unsalted butter. Cut the butter into the dry ingredients using a pastry blender or your fingertips until coarse crumbs form. Place the bowl in the refrigerator while you prepare the cake batter.
Step 2: Preheat Oven & Prepare Pan: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch square baking pan or a 9-inch round springform pan, ensuring all surfaces are lightly coated.
Step 3: Combine Dry Cake Ingredients: In a large mixing bowl, whisk together 1 3/4 cups all-purpose flour, 1 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon salt, and 1 teaspoon ground cardamom until well combined.
Step 4: Combine Wet Cake Ingredients: In a separate medium bowl, whisk together the softened unsalted butter, large eggs, milk, sour cream, and vanilla extract until the mixture is smooth and evenly combined.
Step 5: Combine Wet and Dry Mixtures: Gradually add the wet ingredients to the dry ingredients. Mix on low speed (or by hand with a spatula) until the batter is just combined. Be careful not to overmix; a few small lumps are perfectly normal.
Step 6: Assemble the Cake: Pour the prepared cake batter into your greased and floured baking pan, spreading it evenly across the bottom.
Step 7: Add Crumb Topping: Retrieve the chilled crumb topping from the refrigerator and evenly sprinkle it over the top of the cake batter in the pan.
Step 8: Bake the Cake: Bake for 40-50 minutes, or until a toothpick inserted into the center of the cake comes out with moist crumbs attached, but no wet batter.
Step 9: Cool Before Serving: Remove the cake from the oven and let it cool in the pan on a wire rack for at least 20 minutes before slicing and serving. This allows the cake and crumbs to set properly.