PREPARE YOUR MISE EN PLACE
Position a rack in the center of your oven and preheat to 175°C (350°F). Butter a 9x13-inch baking pan and line the bottom with parchment paper, leaving overhang on the long sides for easy removal. Butter the parchment as well.

Prepare the peaches by bringing a small pot of water to boil. Score a small X on the bottom of each peach and blanch for 30 seconds. Transfer immediately to ice water. The skins should slip off easily. Halve, pit, and dice into 1cm (½-inch) pieces. You should have about 1½ cups of diced peach.

Toss the diced peaches and blueberries together with 1 tablespoon flour in a small bowl. This light coating helps prevent the fruit from sinking to the bottom and absorbs excess moisture during baking.

MAKE THE CRUMBLE TOPPING
In a medium bowl, whisk together the flour, both sugars, cinnamon, and salt. Add the cold cubed butter and work it in with your fingertips, pinching and rubbing until the mixture resembles coarse breadcrumbs with some pea-sized pieces remaining. These larger pieces will create the satisfying crunch that makes this cake so irresistible. Refrigerate while you prepare the batter.

MAKE THE BUTTERMILK SPONGE
Sift the flour, baking powder, and salt together into a bowl. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), beat the softened butter with both sugars on medium-high speed until light and fluffy, about 3-4 minutes. The mixture should be noticeably paler and increased in volume.

Add the egg yolks one at a time, beating well after each addition. Scrape down the sides of the bowl as needed. Beat in the vanilla extract.

Reduce mixer speed to low. Add the flour mixture in three additions, alternating with the buttermilk in two additions, beginning and ending with flour. Mix just until combined after each addition to avoid developing gluten and creating a tough crumb.

In a separate, impeccably clean bowl, whisk the egg whites to stiff but not dry peaks. Using a large rubber spatula, fold one-third of the whites into the batter to lighten it, then gently fold in the remaining whites in two additions. This folding technique creates the incredibly tender, cloud-like crumb that makes this cake special.

ASSEMBLE AND BAKE
Spread the batter evenly in the prepared pan. Scatter two-thirds of the floured fruit mixture evenly over the top, pressing pieces very gently into the batter. Reserve the remaining third.

Sprinkle the crumble topping evenly over the fruit, then scatter the remaining fruit over the crumble. This layering ensures fruit appears both within the cake and beautifully scattered on top.

Bake for 45-50 minutes, until the top is deeply golden, the crumble is crisp, and a skewer inserted into the center comes out clean or with just a few moist crumbs attached. The cake should spring back when gently pressed in the center.

Allow to cool in the pan on a wire rack for at least 20 minutes. Serve warm with crème fraîche, lightly whipped cream, or warm vanilla custard.