Preheat your oven to 300°F and grease a 9x13 inch baking pan. Gather and measure all ingredients, ensuring your butter has reached true room temperature at about 70°F for proper incorporation.

In a medium bowl, whisk together the flour, salt, and baking powder, then set aside. In a large mixing bowl, beat the room-temperature butter with 1 1/4 cups sugar and 2 teaspoons vanilla for 3 minutes until the mixture becomes light and fluffy.

Add the 3 eggs to the butter mixture one at a time, beating for 1 minute after each egg to fully incorporate. Add the flour mixture and milk in alternating portions, beginning and ending with the flour mixture, mixing just until everything is combined without overworking the batter.

Transfer the batter to your greased pan and bake for 45 to 50 minutes, checking doneness with a toothpick inserted in the center—it should come out clean or with only a few moist crumbs. Allow the cake to rest in the pan for 5 minutes before inverting onto a cooling rack to cool completely.

Prepare the lemon curd while the cake cools. In a heatproof bowl, whisk together the 2 egg yolks, 2 whole eggs, 5 tablespoons butter, 2/3 cup sugar, 1/4 teaspoon salt, lemon zest, and fresh lemon juice until well combined.

Set the bowl over a pot of gently simmering water, ensuring the bowl doesn't touch the water surface. Stir constantly for approximately 10 minutes until the curd reaches 166°F and coats the back of a spoon. Strain through a fine-mesh sieve to remove any cooked egg pieces, then let cool to room temperature.

Whip the heavy cream until stiff peaks form—this takes about 5 minutes by hand or less time with an electric mixer. Gently fold the whipped cream into the cooled lemon curd using broad motions to maintain the light, airy texture.

Using a completely clean and dry bowl, beat the 4 room-temperature egg whites with the cream of tartar for about 2 minutes until the mixture becomes foamy. Gradually add the 2/3 cup sugar while continuing to beat for another 3 minutes until stiff, glossy peaks develop.

Cube the cooled cake into 1-inch pieces. In a large trifle dish or 9x13 inch baking dish, create your first layer with cake cubes, followed by a layer of mixed strawberries and raspberries, then a layer of the lemon curd cream. Repeat this layering sequence once more, finishing with any remaining cake on top.

Spread the meringue over the top layer in decorative peaks, ensuring it reaches and touches the dish edges to prevent shrinking. Lightly torch the meringue with a kitchen torch until golden-brown tips appear, or alternatively broil for 1 to 2 minutes under close supervision. Serve immediately or within a few hours for best texture.