Pistachio Celebration Cake With Whipped Custard Cream

Ingredients

• 1/2 cup unsalted butter, softened at room temperature
• 3/4 cup canola or vegetable oil
• 1 ½ cups granulated sugar
• 1 egg plus 4 egg whites, at room temperature, divided (reserve yolks for the custard)
• 2 teaspoons vanilla extract
• 2 ¼ cups shelled pistachios, toasted
• 2 ½ cups all-purpose flour
• 2 teaspoons baking powder
• 1 teaspoon baking soda
• 1 teaspoon kosher salt
• 1 cup buttermilk, divided
• 1/2 cup granulated sugar
• 1/4 cup cornstarch
• 1/8 teaspoon kosher salt
• 4 egg yolks
• 2 cups whole milk
• 2 tablespoons unsalted butter
• 2 teaspoons vanilla extract
• 1 ½ cups heavy whipping cream
• 3/4 cup raspberry or strawberry jam
• 2 tablespoons freshly squeezed lemon juice
• 1/4 teaspoon fine sea salt
• Fresh raspberries or strawberries, to decorate