Step 1Heat the oven to 350 degrees. Grease a 10-inch round cake pan (solid, not a springform) and line the base with parchment paper. Grease the parchment paper and set aside.
Step 2In the bowl of a stand mixer, or in a large bowl using an electric mixer, beat the egg yolks with 5 tablespoons plus 2 teaspoons of the sugar until thick and pale, about 5 minutes. Scrape the seeds from the vanilla pods and beat into the egg mixture. Beat in the yogurt, orange and lemon zest, lemon juice and flour until incorporated.
Step 3In a separate bowl, whisk the egg whites with the remaining sugar until soft peaks form. Gently and evenly, fold the whites into the yogurt mixture. Pour the cake mixture into the prepared cake pan.
Step 4Place the cake pan in a larger roasting pan, allowing for at least 1 inch clearance on all sides. Place the roasting pan in the oven and pour in boiling water halfway up the filled tin.
Step 5Bake the cake for 20 minutes. Quickly but gently sprinkle the pistachios evenly over the top of the cake, then continue baking until the top of the cake is lightly browned, about 20 more minutes. The correct consistency of the cake should be a light sponge on top with a moist custard below. Serve the cake with yogurt.