Preheat your oven to 300°F and grease a 9x13 inch baking pan. Measure out all ingredients and set them in bowls within arm's reach—this is especially important for the eggs, which need to be added one at a time to the butter mixture. I always make sure my butter is truly room temperature (about 70°F) because cold butter won't incorporate smoothly, resulting in a dense cake rather than the tender crumb you want.
Whisk together the flour, salt, and baking powder in a bowl and set aside. In a separate large bowl, beat the room-temperature butter, 1 1/4 cups sugar, and 2 teaspoons vanilla for 3 minutes until light and fluffy—this aerates the batter and is crucial for a tender cake. Add the 3 eggs one at a time, beating for 1 minute after each addition to fully incorporate them. Alternately add the flour mixture and milk to the butter mixture, starting and ending with the flour mixture, mixing just until combined. Don't overmix at this stage or you'll develop too much gluten and toughen the cake.
Pour the batter into your prepared pan and bake for 45-50 minutes, until a toothpick inserted in the center comes out clean or with just a few moist crumbs. Let the cake cool in the pan for 5 minutes, then turn it out onto a cooling rack to cool completely. This short rest prevents the cake from breaking apart, but cooling on the rack allows air circulation underneath so it doesn't steam and become soggy.
While the cake cools, make the lemon curd, which requires gentle heat and constant attention. In a heatproof bowl, whisk together 2 egg yolks, 2 whole eggs, 5 tablespoons butter, 2/3 cup sugar, 1/4 teaspoon salt, lemon zest, and fresh lemon juice. Place the bowl over a pot of gently simmering water (not touching the water), stirring constantly for about 10 minutes until the mixture reaches 166°F and coats the back of a spoon—this ensures food safety while cooking the eggs. Strain the curd through a fine-mesh sieve to remove any cooked egg bits, then let it cool to room temperature. I always make fresh lemon curd rather than using jarred because the bright, fresh flavor is noticeably better in this delicate trifle.
Whip the heavy whipping cream until stiff peaks form (about 5 minutes by hand, less with an electric mixer). Gently fold the whipped cream into the cooled lemon curd—don't stir vigorously or you'll deflate the cream and lose the light, mousse-like texture that makes this filling special.
In a clean, completely dry bowl, beat the 4 room-temperature egg whites with cream of tartar for about 2 minutes until foamy. Gradually add 2/3 cup sugar and continue beating for 3 more minutes until stiff, glossy peaks form. The cream of tartar stabilizes the meringue and helps prevent it from weeping as it sits.
Cut the cooled cake into 1-inch cubes. In a large trifle dish or a 9x13 inch baking dish, begin layering: start with a layer of cake cubes, then a layer of mixed berries (strawberries and raspberries), then a layer of the lemon curd cream. Repeat the layers one more time, ending with any remaining cake. Top with the meringue, spreading it in decorative peaks and making sure it touches the edges of the dish so it won't shrink. Use a kitchen torch to brown the meringue lightly until it has golden-brown tips, or place it under the broiler for 1-2 minutes, watching carefully so it doesn't burn. Serve immediately or within a few hours—the trifle is best enjoyed while the meringue is still crispy and the cake hasn't absorbed too much moisture.
