Preheat oven to 375°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
Brown the butter: In a small saucepan over medium heat, melt the cubed butter. It will foam, then settle. Swirl the pan until the butter turns a deep amber color and smells like toasted nuts. Pour into a bowl and let cool slightly.
Make the crust and streusel: In a food processor, pulse flour, brown sugar, and salt. Add browned butter and pulse until mixture looks like wet sand. Reserve about 1 cup for the topping. Press the remaining mixture firmly into the prepared pan using the bottom of a measuring cup.
Pre-bake the crust: Bake for 12 minutes, until the edges are just starting to turn golden. Remove from oven.
Prepare the filling: In a bowl, toss huckleberries with granulated sugar, cornstarch, lemon juice, and vanilla. If using frozen berries, do not thaw.
Assemble: Spread the berry mixture evenly over the hot crust. Sprinkle reserved crumb mixture over the top in clumps.
Bake: 30-35 minutes, until the top is deep golden brown and berries bubble around edges. If top browns too fast, tent with foil for last 10 minutes.
Cool: Let bars cool completely in the pan on a wire rack, at least 2 hours. Cutting them warm will cause crumbles.