From Salt Water Farm via Bon Appétit, with changes. Rewritten in my words and with weight measures.
The cake is damp. It is soft. It is held together by its crust, and once it's broken, all bets are off. It is not one to cut neatly. Yet, it is staggeringly sublime as is, eaten out of hand in unstable chunks, or with a spoon and a mound of crème fraîche or a lick of cream or custard. It is a buttery muffin-meets-cobbler-meets-coffeecake kind of thing. It is custardy where cake meets fruit, and crunchy where there is streusel, which is to say, a buckle for cheering. I think it would be a dream with raspberries. Or nectarines. Or both.
FOR THE TOPPING
1/2 cup (100 g) granulated sugar
1/4 cup (32 g) all-purpose flour
1/2 teaspoon ground cinnamon
1/4 teaspoon kosher salt
1/8 teaspoon ground cardamom
1/8 teaspoon ground ginger
1/4 cup (57 g) unsalted butter, cold and diced
FOR THE CAKE
1/4 cup (57 g) unsalted butter, plus more for the pan
1 1/2 cups (191 g) all-purpose flour, plus more for the pan
2 teaspoons baking powder
3/4 teaspoon kosher salt
3/4 cup (150 g) granulated sugar
1 egg, room temperature
2 teaspoons vanilla extract or seeds scraped from a vanilla bean
1/4 teaspoon almond extract
1/2 cup (120 ml) heavy cream
10 ounces (283 g) pitted cherries, I used a mix of tart and sweet
6 ounces (170 g) blueberries, fresh or defrosted
METHOD
Start with the topping. Whisk sugar, flour, and spices in a medium bowl. Tumble in the butter cubes and rub between your fingers until the mixture is evenly damp and coming together in clumps. Set aside.
For the cake, preheat an oven to 350°F / 175°C. Grease an 8-inch springform or removable bottom pan. Line the base of the pan with parchment, then grease the parchment. Dust the pan with flour, and tap out the excess.
Whisk the 1 1/2 cups flour, baking powder, and salt in a medium bowl.
In another medium bowl, beat the butter and sugar together with an electric mixer on high speed until light and fluffy, around 5 minutes. Add the egg, vanilla, and almond extract and beat to combine, 2 minutes. Turn the speed down to low and gradually add the dry ingredients, stirring until mostly incorporated. Pour in the cream and stir until smooth. With a spatula, fold in the cherries and blueberries.The batter will be quite thick, and may not fold easily; as long as the fruit is somewhat stuck into the batter, all will be fine. Scrape the batter into the prepared pan, and smooth the top. Place tin on a rimmed baking sheet, then sprinkle the topping over the batter in an even layer.
Bake in the hot oven until the buckle is golden brown and a cake tester poked into the centre comes out clean, 75-90 minutes. Transfer pan to a wire rack and let the cool completely. Unmold and serve, as is, or dusted with icing sugar, and maybe a spoon or two of custard.
Note: I think this buckle would be ideal baked in individual portions, thus dispensing of any fuss of slicing. I've not tried that route, but wanted to have the notion on record.