In a pot, whisk together the sugar, rice flour, and salt until combined, then gradually whisk in the room temperature milk until completely smooth with no lumps. Heat the mixture over medium-low heat, stirring occasionally, until it reaches a gentle simmer. In a separate bowl, whisk the egg yolks together. Slowly pour about 1/2 cup of the hot milk mixture into the egg yolks while whisking constantly to temper them and prevent scrambling. Pour the tempered egg mixture back into the pot with the remaining milk and whisk continuously over medium-low heat for about 3 minutes until the custard thickens enough to coat the back of a spoon. Strain the custard through a fine sieve into a clean bowl to remove any lumps, then stir in the vanilla essence. Press plastic wrap directly onto the surface of the custard to prevent a skin from forming, then let it cool to room temperature before transferring to the refrigerator for at least 2 hours. I like to use rice flour because it creates a silkier, smoother custard texture compared to cornstarch.
While the custard chills, cut the pound cake into 1-inch cubes and set aside. Wash and thoroughly dry the strawberries, then hull them and slice into 1/4-inch pieces. Set aside about 20 of the best-looking strawberry slices to press against the glass edges of the trifle bowl for visual appeal. Finely chop or mash the remaining strawberries and toss them with the white wine vinegar and lemon zest—this maceration will draw out the berries' juices and create a flavorful berry mixture that will layer beautifully throughout the trifle.
In a large trifle bowl or similar glass serving dish, begin with a layer of pound cake cubes covering the bottom. Spread half of the cold custard from Step 1 evenly over the cake. Press the reserved strawberry slices against the inner glass of the bowl around the sides so they're visible. Add half of the macerated strawberries and about half of the blueberries over the custard layer. Top with another layer of pound cake cubes, then spread the remaining custard over them. Again, press strawberry slices against the glass sides, then add the remaining macerated strawberries and most of the remaining blueberries. I find that pressing the fruit against the glass creates a gorgeous presentation—guests can see all the beautiful colors and layers before you serve it.
Cover the trifle bowl with plastic wrap or aluminum foil and refrigerate for at least 2 hours. This resting time allows the flavors to meld and the cake to absorb some of the berry juices and custard, creating a more cohesive and flavorful dessert.
Just before serving, remove the trifle from the refrigerator. In a chilled bowl, combine the whipping cream and cream of tartar. Whip on medium speed until soft peaks form, then add the sifted powdered sugar and continue whipping until stiff peaks form and the cream holds its shape. I prefer Horizon Organic cream because it whips to a more stable peak and holds up better without deflating.
Spread or dollop the whipped cream generously over the top of the chilled trifle, then scatter the remaining blueberries over the cream for a final touch of color and freshness. Serve immediately from the bowl, allowing guests to see the beautiful layers as you scoop.
