Instructions

For the cake: Adjust oven rack to middle position and heat oven to 350 degrees. Grease and flour light-colored 9-inch springform pan. Place 3 cups thawed frozen huckleberries in colander and let drain while making streusel, at least 10 minutes.
For the streusel: Meanwhile, stir 1 cup (5 ounces) all-purpose flour, ½ cup packed (3½ ounces) light brown sugar, 6 tablespoons melted unsalted butter, and ½ teaspoon table salt in bowl until no dry spots remain and mixture forms clumps. Refrigerate until streusel is firm, at least 10 minutes. Keep refrigerated until ready to use.
Whisk ½ cup whole milk, 2 large eggs, 4 tablespoons melted unsalted butter, and 1 teaspoon vanilla extract in medium bowl until well combined. Whisk 1 cup (5 ounces) all-purpose flour, ¼ cup (1¾ ounces) granulated sugar, 1½ teaspoons baking powder, 1 teaspoon grated lemon zest, and ½ teaspoon salt together in large bowl. Stir milk mixture into flour mixture until just combined.
Pat huckleberries dry with paper towels. Toss berries with remaining ¼ cup (1¾ ounces) granulated sugar in second medium bowl until coated. Using silicone spatula, gently fold half of berry mixture into batter until evenly distributed. Transfer batter to prepared pan and spread to edges of pan with spatula. Sprinkle remaining half of berry mixture evenly over top.
Break streusel into pea-size crumbs and distribute evenly over berries. Bake until top of buckle is golden brown and toothpick inserted in center comes out clean, about 50 minutes, rotating pan halfway through baking. Let buckle cool in pan on wire rack for 1 hour.
Run paring knife around edges of pan to release buckle from pan. Remove side of pan. Slide thin metal spatula between buckle and pan bottom to loosen, then slide buckle onto serving platter. Cut into wedges and serve.