In a small bowl, combine the topping ingredients until fully blended. In a separate bowl, whisk together the icing ingredients and set aside until needed.
Preheat the oven and prepare the pan
Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with nonstick spray. In a medium bowl, sift together the flour, salt, baking soda, cinnamon, and baking powder.
Combine wet ingredients
In a large mixing bowl, whisk together the eggs, melted butter, buttermilk, brown sugar, and granulated sugar until smooth and well incorporated.
Mix wet and dry ingredients
Gradually add the dry mixture to the wet mixture. Stir until just combined; the batter will be thick.
Incorporate flavor additions
Fold in the lemon zest and blueberries gently, ensuring they are evenly distributed throughout the batter. Transfer the batter into the prepared pan, spreading it evenly.
Add topping and bake
Sprinkle the prepared topping mixture evenly across the surface of the batter. Bake for 40–50 minutes, or until a toothpick inserted into the center comes out clean.
Cool and glaze
Place the cake on a cooling rack for 20 minutes. If the icing has thickened, stir again and add a touch of milk if necessary. Drizzle the icing evenly over the cooled cake before serving.
Notes
To make this cake gluten-free, substitute the all-purpose flour with a certified gluten-free flour blend suitable for baking. Ensure all other ingredients, particularly the baking powder and confectioner’s sugar, are labeled gluten-free.