In a medium saucepan over medium-high heat, whisk the half-and-half, the 1/3 cup of sugar, the cornstarch and a pinch of salt until it comes to a boil. In a medium bowl, whisk 2 egg yolks, then slowly whisk in half of the hot mixture. Pour the custard back into the pan and cook, whisking, until it comes to a boil. Transfer to a bowl and stir in 1 teaspoon of the vanilla. Cover with wax paper and refrigerate for 1 hour.
Meanwhile, preheat the oven to 350 degrees . Lightly butter a 9-inch square cake pan, line it with wax paper, then butter the paper.
In the bowl of a mixer fitted with a whisk, combine the 4 remaining yolks, the remaining 1 teaspoon of vanilla and the 3/4 cup of sugar. Beat on high speed until pale and thick, 3 minutes. In a small saucepan, bring the milk almost to a boil and, with the mixer at low speed, drizzle it into the yolk mixture. Mix in the flour until just blended. Transfer to a bowl and clean the mixer bowl and whisk.
Melt the butter until just warm. Place the 4 egg whites, cream of tartar and 1/2 teaspoon of salt in the mixer bowl. Whisk on medium-high speed until stiff but not dry, about 3 minutes. Fold one-third of the whites into the yolk mixture, then fold in the remaining whites and the butter.
Scrape the batter into the prepared pan; bake until a toothpick inserted in the center comes out clean, about 30 minutes. Cool in the pan on a rack for 5 minutes, then turn out and let cool completely.
With a serrated knife, divide the cake into 2 layers. Place a layer on a cake stand; top with the custard and then the other layer. Melt the jelly in a microwave and lightly brush over the cake. Arrange the fruit on the cake and brush with more jelly.