Sam's cake is as worthy a pedestal for your prized summer berries as pie. Because her real innovation—besides turning tres leches into cuatro leches (cream, evaporated milk, condensed milk, and, fourthly, whole milk)—is cramming halved blackberries over the top.
The high-rising cake eclipses the berries, which relax into the batter below. When the cake comes out of the oven, you poke holes all over its top and drown it in the sweet milk. It pools in the blackberries' bellies and fills all of the cake's pockets.
Chill the cake for four hours, enough time for it to become pudding-soft. Then spoon more leche on top and scatter over any lingering blackberries. You've got a cake, a custardy blackberry pie, and a pudding all in one. —Sarah Jampel
Preheat the oven to 350° F and butter an 8- by 8-inch baking dish. In a small bowl, whisk together the flour, baking powder, and salt.
In the bowl of a stand mixer, or working with an electric hand mixer, beat together the egg yolks and 1/2 cup of sugar on medium speed until very pale and thick, 3 to 4 minutes. Meanwhile, halve the berries lengthwise. Now, beat in 1/3 cup of milk and the vanilla bean seeds. Add the flour mixture and mix until just combined.
In a large bowl with clean beaters (transfer the batter to another bowl if necessary), whip the egg whites for about 1 minute, until foamy and white. With the mixer running, slowly add the remaining 1/4 cup sugar; continue to beat until you have shiny, medium-stiff peaks, about 2 minutes. Stir about 1/4 of the egg white mixture into the batter, to loosen it up a bit, then use a rubber spatula to fold the remaining white into the batter.
Transfer to prepared pan and place the halved berries on top. Bake until golden brown and a toothpick inserted comes out clean, 35 to 40 minutes. Let cool for 5 minutes while you mix together the milk mixture.
In a small bowl, stir together all the ingredients for the milk soak. Use a butter knife to cut around the edges of the cake. Then use a toothpick or a fork to make holes all over the top of the cake. pour about 1 cup of the milk mixture all over the cake and let it soak in. Then pour another cup over the cake. Cover and chill for at least 4 hours or up to overnight.
Serve the cake with remaining mixture and more blackberries. Leftovers will keep, well-wrapped, in the fridge for up to 2 days.
A (former) student of English, a lover of raisins, a user of comma splices. My spirit animal is an eggplant. I'm probably the person who picked all of the cookie dough out of the cookie dough ice cream. For that, I'm sorry.