Preheat the oven to 350° F. Line a 9 or 10-inch springform pan with parchment paper, butter the parchment paper.To make the cake. In a large mixing bowl, beat together the butter, 1/2 cup sugar, and lemon zest until light and fluffy, about 2 minutes. Beat in the egg and 1 tablespoon vanilla until combined. Add the flour, poppyseeds, baking soda, and salt, and beat to combine. Beat in the sour cream until just combined. Spread the batter over the bottom and 1 1/4 inch up sides of the prepared pan, creating a boat for the custard. Making sure batter on sides is 1/4 inch thick and you have an even layer of batter on the bottom (see above photos).To make the custard. Whisk together the egg yolks and 2 tablespoons sugar in a medium bowl until combined.Place the cream over medium heat and gently warm until steaming. Remove from the heat and stir in the vanilla. In a slow stream, whisk about 1/2 cup of the cream into the egg yolks, whisking constantly so they don’t scramble. Whisk in the remaining cream. Pour 3/4 of custard into the center of the cake, reserving the remaining 1/3 cup. In a bowl, combine the blueberries, sugar, and flour. Sprinkle 1 1/2 cups of berries over the custard. It's going to be jiggly. Bake for 25 minutes. Remove from the oven and gently pour over the remaining custard, then sprinkle on the remaining berries. Bake another 20 - 25 minutes until the sides of the cake are deep golden. The custard should be pretty jiggly in the center. If you want a looser custard, bake a total of 45-48 minutes.Let cool, then release the cake from the mold. Refrigerate 1-2 hours or until ready to serve. The colder your cake is, the more set your custard will be. When ready to eat, generously dust the top of the cake with powdered sugar and enjoy!!