Heat your oven to 375°F and generously grease a 9x13 inch baking dish or similar size casserole dish with butter or cooking spray. This ensures your buckle won't stick and will have nice edges. Have all your ingredients measured and ready before you begin mixing—this is especially important since the batter comes together quickly.

In a medium bowl, cream together the softened 1/3 cup butter and 1/2 cup sugar until the mixture is light and fluffy, about 2-3 minutes of mixing. This incorporates air into the batter, which helps the cake rise. In a separate small bowl, whisk together the flour, baking powder, salt, and lemon zest. Alternate adding the dry mixture and milk to the butter mixture, starting and ending with the dry ingredients—this prevents overmixing and ensures a tender crumb. Stir in the vanilla extract and mix just until combined. I like to add the lemon zest to the dry ingredients rather than the liquid so it distributes evenly throughout the batter.

Pour the batter from Step 2 into your prepared baking dish, spreading it gently to create an even layer. In a small bowl, combine the 2.5 cups huckleberries with the 3/4 cup sugar and let it sit for about a minute so the berries begin to release their juices. Pour the huckleberry mixture (including all the liquid) over the batter—don't stir it in, as the berries will sink and create pockets throughout as it bakes. Dot the top with small pieces of the 1 tablespoon butter, distributing them evenly across the surface.

Place the buckle in your preheated 375°F oven and bake for 45-50 minutes, until the top is golden brown and a toothpick inserted into the cake portion (avoiding the berry pockets) comes out clean or with just a few moist crumbs. The berries will cook down and create a beautiful sauce that mingles with the cake, while the top will have a slightly crispy, golden crust. I like to check it around the 40-minute mark since ovens vary—you want the edges set and the top nicely browned but not darkened.

Remove the buckle from the oven and let it cool for at least 15 minutes before serving. This cooling time allows the structure to set while the dessert is still warm and the berry sauce is at its best consistency. Serve directly from the baking dish with a spoon, scooping through the cake and berry layers together to give each serving some of the jammy huckleberry sauce.