Preheat oven to 375°F. Line an 8-inch square baking pan with parchment paper.
In the bowl of a stand mixer fitted with a paddle attachment, beat the sugar and butter together until combined and creamy. Beat in the egg.
In a bowl, combine the flour, baking powder, salt and lemon zest. Add to the mixer in 3 additions, alternating with the milk and mixing after each. Fold in the huckleberries. The batter will be thick. Spread into the prepared baking pan.
To prepare the topping, combine the brown sugar, flour, cinnamon, and melted butter in a small bowl. Sprinkle over the cake batter.
Bake until a toothpick inserted into the center comes out clean with a few moist crumbs and the topping is golden brown, 30-35 minutes. Let cool completely, then slice and serve.
Calories: 209kcal, Carbohydrates: 35g, Protein: 3g, Fat: 7g, Saturated Fat: 4g, Cholesterol: 27mg, Sodium: 87mg, Potassium: 119mg, Fiber: 1g, Sugar: 19g, Vitamin A: 227IU, Vitamin C: 2mg, Calcium: 51mg, Iron: 1mg