1. Preheat the oven to 350°F (175°C). Grease the bottom and sides of a 9-inch springform pan with butter or cooking spray. If you’re worried about sticking, you can also line the bottom with parchment paper

Make the crumble:

1. In a small bowl, whisk together ⅔ cup (85 g) all-purpose flour, 3 tablespoons (40g) packed light brown sugar, 2 tablespoons (25g) granulated sugar, and a pinch of kosher salt. Pour in 4 tablespoons (57 g) melted unsalted butter and stir until the mixture is evenly moistened. It should look like wet sand and hold together in small clumps when squeezed. Refrigerate until ready to use.

Make the honey buckle batter

1. In a large bowl, rubbed together 1 tablespoon lemon zest and ¼ cup granulated sugar to help release the lemon oil. Whisk in ½ cup whole milk, 6 tablespoons melted unsalted butter, ⅓ cup honey, 2 large eggs, 1 teaspoon vanilla extract, and 1 tablespoon lemon juice until smooth and well combined.
2. Sprinkle the 1½ teaspoons baking powder, ½ teaspoon kosher salt, and 3/4 teaspoon ground ginger over the mixture and whisk to combine. Add 1½ cups all-purpose flour and rubber spatula, gently fold until just incorporated—stop as soon as you no longer see streaks of dry flour. A few lumps are fine. (Overmixing can develop gluten and make the cake tough.
3. Spread the batter evenly into the prepared pan; it will form a shallow layer.

Make the blueberry layer: 

1. In the same bowl you made the batter, toss together 1 pint fresh blueberries, 3 tablespoons finely chopped crystallized ginger, 2 tablespoons granulated sugar, and a pinch of kosher salt. 

Assemble:

1. Sprinkle the blueberry mixture evenly over the top and gently press it into the batter.  Sprinkle the chilled crumble over the top, using a mix of small and larger clumps for texture.
2. Place the springform on a baking sheet (to catch any drips and make it easier to move) and bake for 40–50 minutes, rotating the pan halfway through.
3. The buckle is done when a toothpick or cake tester inserted in the center (avoiding juicy berry spots) comes out with moist crumbs but no wet batter. For accuracy, the center should register 200–205°F (93–96°C) on an instant-read thermometer.
4. Let the buckle cool in the pan for 10 minutes. Run a thin knife around the edge, then release the springform ring. Cool at least 1 hour  before serving.  The flavor and texture improve as the cake sets.
5. Optional: Serve with drizzle of honey, sprinkle of flaky sea salt and/or dollop of whipped cream