Topping:
1/3 cup packed light brown sugar
1/3 cup granulated sugar
1 tbsp. cinnamon
1 tbsp. unsalted butter, melted and cooled
1 cup pecans, almonds, or walnuts, chopped coarse
Cake:
3 cups plain flour
1 tbsp. baking powder
1 tsp. baking soda
1 tsp. cinnamon
1/4 tsp. salt
1 3/4 cups buttermilk or plain yogurt
1 cup packed light brown sugar
1 cup granulated sugar
3 large eggs
7 tbsp. unsalted butter, melted and cooled
For the topping:
Mix the brown sugar, granulated sugar, cinnamon, and melted butter together in a medium bowl until the mixture resembles wet sand. Stir in the nuts.
Adjust an oven rack to the middle position and heat the oven to 180 degrees C/ Gas mark 4. Generously coat a 9x13 baking pan with vegetable oil spray (or butter).
For the cake:
Mix the flour, baking powder, baking soda, cinnamon, and salt together in a large bowl. Whisk the buttermilk, brown sugar, granulated sugar, eggs, and melted butter together in a separate bowl until smooth. Gently fold the egg mixture into the flour mixture with a rubber spatula and stir until the batter looks smooth and well combined.
Scrape the batter into the prepared pan and smooth the top. Sprinkle the streusel evenly over the batter. Bake until the top is golden and a toothpick inserted into the center comes out with just a few crumbs attached, 40 or 50 minutes (depending on your oven).
