FOR THE BLUEBERRY PUREE
3 cups (420 grams) fresh or frozen blueberries, plus a handful of fresh for decorating, if desired
3/4 cup (180 milliliters) water
1/2 cup (100 grams) granulated sugar
1 tablespoon cornstarch
1/8 teaspoon fine salt
2 teaspoons fresh lemon juice
FOR THE VANILLA CAKE
Cooking spray or softened butter, for greasing the pan
1 2/3 cups (333 grams) granulated sugar
1/2 cup (120 milliliters) vegetable oil
1/2 cup (4 ounces/113 grams) unsalted butter, melted and slightly cooled
1 tablespoon vanilla extract
2 large eggs, at room temperature
1 large egg yolk, at room temperature
1 3/4 cups (420 milliliters) buttermilk, at room temperature
2 teaspoons baking powder
3/4 teaspoon baking soda
1 teaspoon fine salt
2 2/3 cups (308 grams) cake flour
FOR THE WHIPPED CREAM
2 cups (480 milliliters) heavy cream
3 tablespoons confectioners’ sugar
2 teaspoons vanilla extract
Make the blueberry puree: In a medium saucepan over high heat, stir together the blueberries, water, sugar, cornstarch and salt. Bring to a boil and cook, stirring occasionally, for about 5 minutes. Reduce the heat to medium-high and simmer until the puree thickens and the berries begin to break down, about 15 minutes, adjusting the heat as needed to maintain a gentle simmer. Remove from the heat and let cool for about 5
Transfer the puree to a blender and blend on medium-high speed until thick and pourable. Pour the puree into a 2-cup liquid glass measuring cup with a spout, if you have one, or a spouted pitcher, and mix in the lemon juice. Set aside 3 to 4 tablespoons of puree in a small bowl.
Make the cake: Position the rack in the middle of the oven and preheat to 350 degrees. Grease a 9-by-13-by-2-inch baking pan with cooking spray or butter. Line the pan with a sheet of parchment paper with a generous overhang on the long sides.
In a large bowl, whisk together the sugar, oil, butter and vanilla until thick, glossy and thoroughly combined, about 30 seconds. Whisk in the eggs and yolk, one at a time, ensuring each is incorporated before adding the next. Whisk in the buttermilk, followed by the baking powder, baking soda and salt, one at a time, vigorously whisking after each addition. Using a sifter or fine-mesh strainer, sift the flour over the bowl and whisk until almost smooth; a few small lumps are okay.
Scrape the batter into the prepared pan, and lightly drop the pan on the counter a few times to pop any air bubbles. Bake for 30 minutes, rotating the pan halfway through, until a cake tester comes out with a moist crumb or two, the cake bounces back when lightly pressed with your finger and is just starting to come away from the sides of the pan.
Transfer the cake pan to a wire rack and immediately begin poking the cake with the end of a wooden (or silicone) spoon (see headnote). The holes should be about 1 inch apart and should evenly cover the entire top of the cake. Press the spoon handle into the cake until it feels like you may have hit the bottom of the pan. Depending on your poking utensil, you may need to wipe off the end with a damp cloth after creating each hole, as the cake crumbs may stick to it. If some of the holes (or all of them) look like they’ve closed up a bit after you’ve poked, gently re-poke them. Spoon or drizzle the puree into the holes, filling each one up. The cake absorbs the puree quickly, so refill the holes with the puree until you run out, or until the cake seems adequately filled. (see NOTE)
Refrigerate the cake, still in its pan and on the cooling rack, for about 2 hours, or until the bottom of the pan is cold to the touch. Or, if you’re feeling impatient, freeze for 30 to 45 minutes.
Make the whipped cream: When ready to serve, in the bowl of a stand mixer fitted with the whisk attachment — or, if using a hand-mixer, in a large bowl — combine the cream, sugar and vanilla and beat on medium to medium-high speed until medium to stiff peaks form, about 5 minutes.
Run a butter knife around the short ends of the pan (the ones without the parchment overhang) and, holding on to the parchment overhang, lift the cake out of the pan.
Place the cake on a rectangular serving platter or cutting board. Because it’s nice to slice the cake through the rows of holes, for optimal berry exposure, eyeball where the holes are (or make tiny little marks with a paring knife,) before generously frosting the top of the cake with the whipped cream, leaving the sides naked. Drizzle some of the reserved puree on top of the whipped cream and drag your spoon through it to create streaks. Your goal is a white whipped cream with purple berry swirls
Sprinkle a few blueberries over the cake, if desired, slice along the rows of holes and serve.
Note: This is a variation of a trifle.
You can use a can of pie mix for the fruit filling.
You can use vanilla INSTANT pudding mix For the topping but substitute heavy whipping cream for the milk called for on the INSTANT pudding mix box and it makes the equivalent of a pastry cream for a frosting on top. Serve with whipped cream optional on the side.