Preheat your oven to 375°F (190°C) and line an 8-inch square baking pan with parchment paper for easy removal.

Using a stand mixer fitted with the paddle attachment, cream together the softened butter and granulated sugar until the mixture is light and fluffy. Add the egg and beat until well combined.

In a separate bowl, whisk together the all-purpose flour, baking powder, salt, and lemon zest. Gradually add this dry mixture to the creamed butter and sugar in three parts, alternating with the whole milk. Mix after each addition until just combined. Gently fold in the huckleberries, being careful not to crush them. The batter will be quite thick. Spread the batter evenly into the prepared baking pan.

For the topping, mix together the brown sugar, flour, and cinnamon in a small bowl. Pour in the melted butter and stir until the mixture resembles coarse crumbs. Sprinkle this topping evenly over the cake batter.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean with just a few moist crumbs attached, and the topping is golden brown. Allow the cake to cool completely in the pan on a wire rack. Once cooled, slice into squares and serve.