Put the flour in a large bowl. Add the sugar and mix. Make a well in the center and place the butter and egg yolk in the well. Using your fingers, work the butter and egg yolk together until well combined, then work the butter-egg mixture into the flour-sugar mixture until combined but still loose. The dough will be crumbly, but when you squeeze a handful, it should hold together.

Sprinkle the dough into a 10-inch pie or tart pan. Press the dough firmly and evenly onto the bottom and sides of the pan, making sure not to let the crust thicken at the corners. It should make a clean right angle where the sides meet the bottom. Chill the crust at least 30 minutes, or covered, up to 3 days. Heat the oven to 400 degrees. Bake until light golden brown, 20 to 25 minutes. Let cool before filling.

Blueberry Bavarian Cream: Pick over the blueberries, setting aside the best ones to make a circle around the edge of the finished tart. (You'll need about 1 cup.) Heat the milk and vanilla bean in a saucepan over medium heat just until it boils. Immediately turn off the heat and set aside to infuse for 10 minutes. Combine the gelatin and water in a small bowl, and let dissolve, stirring occasionally, until no dry spots remain.

Whisk the egg yolks and sugar together in a bowl. Whisking constantly, slowly pour the still-hot milk mixture into the egg yolk mixture. Return the mixture to the saucepan and cook over medium heat, stirring constantly with a wooden spoon. At 160 degrees, the mixture will give off a puff of steam. When the mixture reaches 180 degrees it will be thickened and creamy, like eggnog. If you don't have a thermometer, test it by dipping a wooden spoon into the mixture. Run your finger down the back of the spoon. If the stripe remains clear, the mixture is ready; if the edges blur, the mixture is not quite thick enough. When it is ready, quickly remove it from the heat.

Meanwhile, fill a large bowl with ice water half way. Strain the mixture into a medium bowl to smooth it and remove the vanilla bean. Pour off 1 cup of this mixture (Anglaise) into a large bowl and set aside. Place the medium bowl containing the remainder of the mixture in the ice water and let cool, stirring often. Add the gelatin to the cup of hot Anglaise in the large bowl and mix until completely dissolved, creating a Bavarian cream. Working quickly, fold the larger amount of berries into the Bavarian cream and spoon the mixture into the center of the tart shell. Spread until smooth and even. Before the filling cools completely, arrange the reserved berries around the edge of the tart to form a rim. Chill at least 2 hours and serve in slices, pouring a bit of the Anglaise next to each side.