1-First, toast the hazelnuts by spreading them on a baking sheet and baking at 300°F for 5-7 minutes until hot and fragrant. Cool them completely, then chop finely for the cake and roughly for the topping. Preheat your oven to 350°F and position the rack in the center, then lightly spray a 9-inch square baking pan with non-stick baking spray.

2-Next, make the batter by whisking together the flour, finely chopped hazelnuts, salt, baking soda, cinnamon, cardamom, and espresso powder in a bowl. In another bowl, whisk the brown sugar, eggs, sour cream, melted butter, oil, and vanilla until combined. Add the wet ingredients to the dry ones and fold gently with a spatula until just moistened, then spread the batter evenly in the prepared pan.

3-For the crumb topping, combine the flour, granulated sugar, brown sugar, cinnamon, salt, and roughly chopped hazelnuts in a bowl. Stir in the melted butter until the mixture is crumbly, then sprinkle it evenly over the cake batter. Bake for 35-40 minutes until the topping is golden brown and a toothpick inserted in the center comes out clean.

4-After baking, cool the cake on a rack for about 1 hour. Serve it warm or at room temperature for the best flavor. Preparation time is simple: prep takes 10 minutes, cooking is 35 minutes, and total time is 45 minutes, making it ideal for quick baking sessions. For more ideas, explore resources like high altitude baking tips to refine your skills.