Step 1In the bowl of an electric mixer fitted with a whisk attachment, beat the egg yolks and sugar on high until the mixture is very thick, pale yellow and forms a ribbon when the beater is lifted from the bowl, about 3 minutes. Remove the bowl from the mixer and sift in the cornstarch and flour, whisking to combine.
Step 2In a medium stainless steel saucepan over high heat, begin to warm the milk. Add the vanilla bean and seeds. Bring the milk to a boil. Remove the vanilla bean. Slowly pour about one-fourth of the hot milk into the egg mixture, whisking constantly. Return the egg-milk mixture to the saucepan and combine with the remaining milk.
Step 3Cook over medium heat, whisking the mixture until it's thickened and bubbles in the center, about 6 minutes. Over a medium bowl, strain the cream through a fine mesh sieve. Cover the mixture with plastic wrap, pressing it against the surface of the cream to prevent a skin from forming. Place the bowl over ice while you prepare the filling.
Step 1Prepare the filling. In the bowl of a food processor fitted with the steel blade, grind the almonds with half of the powdered sugar until it's the consistency of a fine meal.
Step 2In the bowl of an electric mixer, cream the butter on low, 2 to 3 minutes, until softened. Add the remaining powdered sugar and mix on medium 3 to 4 minutes until light and fluffy, scraping down the sides of the bowl as needed. Add the almond mixture and mix another minute, until combined.
Step 3In a medium bowl, whisk together the egg, egg yolk, rum and almond extract. Turn the mixer to low and slowly add the egg mixture a few teaspoons at a time into the butter mixture, mixing until incorporated. Add the pastry cream in batches and mix on low until just combined. Refrigerate until chilled, about an hour. Let the puff pastry sheets temper in the refrigerator 1 hour before rolling.
Step 4On a lightly floured surface, quickly roll one sheet of puff pastry dough into a 12-inch circle, one-eighth-inch thick, flouring the surface of the dough as necessary.
Step 5Lightly coat a 10-inch tart ring with melted butter. Place the ring on a parchment-lined baking sheet. Gently fold the circle of dough into quarters. Position the point in the center of the mold and carefully unfold. (If the dough is too soft to move, chill it on a baking sheet until firm enough to handle. Alternately, if it is too rigid to handle, let it warm for a few minutes to soften.) Fit the dough into the pan by working your way around the edges, gently lifting up the dough and easing it down so that it fits into the corners and sides of the mold. Dip the knuckle of your index finger in flour and run it around the inside of the pan, gently pressing the dough into the corners with the flat part of the knuckle. Using the three middle fingers of one hand, press the dough into the sides of the pan, pinching slightly if necessary to make sure that the dough comes up slightly above the top of the rim and is an even thickness all around. Chill until firm, 30 minutes to 1 hour.
Step 6Flouring the surface as necessary, roll out the second sheet of puff pastry and cut out an 11-inch circle to make a top crust. Place it on a baking sheet, and using a straight-edge razor or a very sharp knife, score the dough with diagonal lines, spaced three-fourths inch apart, to cover the entire surface, being careful not to cut all the way through. Score diagonal lines going in the other direction to make a diagonal grid. Chill until firm, 30 minutes to an hour.
Step 7Adjust the oven rack to the middle position. Heat the oven to 350 degrees. Remove the tart shell and the top crust from the refrigerator. Spoon the filling into the tart shell and spread evenly. Press the excess dough down over the outer rim of the ring. Brush the edge of the tart shell with the egg white. Center the top crust over the tart, pressing down gently. Using a rolling pin, roll over the edge gently to seal the top crust to the bottom crust and cut away the excess dough. Brush the top crust with the egg white and sprinkle the surface with the sugar.
Step 8Bake for 1 hour, 20 minutes or until nicely browned. Remove from oven. Remove the tart ring and let the tart cool to warm. Increase the oven temperature to 400 degrees to roast the cherries while the tart is cooling.
Step 1In a large bowl, toss together the cherries, sugar, oil, kirsh and vanilla bean and seeds. Place the mixture in a baking dish.
Step 2Bake for 7 to 12 minutes, stirring halfway though. Be careful not to over-bake; the cherries should wilt and release some juices but remain intact. Keep warm until ready to serve.
Step 3Slice the gateau with a long serrated knife starting at the side and working to the center gently, sawing back and forth with gentle pressure. Cut into 10 portions and place on individual plates. Spoon the warm roasted cherries and their juices onto the plates, surrounding the cake. Serve immediately.