Preheat oven to 350 degrees.
In a nonstick saucepot, bring the water, milk, butter, sugar, and salt to a boil. Add the flour and using a wooden spoon, mix until mixture is smooth. Continue to cook over low heat for about 3-5 minutes, stirring constantly so it does not dry out, and transfer to a KitchenAid fitted with a paddle attachment. Mix on low speed and add 8 of the eggs, 1 at a time until incorporated and mixture is smooth. Transfer pate a choux to a pastry bag fitted with a star tip and pipe 4-inch in diameter circles onto a silpat.
In a small bowl, combine the remaining egg and the sugar and brush pate a choux with egg wash and sprinkle the top of each ring evenly with chopped almonds and grain sugar. Bake for 20-25 minutes, or until deep golden and firm to the touch. Allow to cool completely on a rack. Halve pastry horizontally with a serrated knife and invert the top onto a work surface.
In a saucepot set over medium heat combine the milk, ¼ cup of the sugar, and egg yolks. Cook over high heat, stirring constantly with a rubber spatula, until mixture reaches 183 degrees (similar to a crème anglaise). Pour mixture into a bowl of a KitchenAid fitted with the whisk attachment and whisk over high speed until mixture cools to room temperature. Decrease speed to low and add the butter and whisk until combine. Remove from bowl and reserve for later use.
Add the egg white to a clean bowl in a KitchenAid fitted with the whisk attachment and whisk until mixture forms soft peaks. Set aside.
In a saucepot set over medium heat combine the remaining ¼ cup sugar and 1 tablespoons of water and cook until mixture reaches 250 degrees. Pour over the whipped egg whites and mix over medium speed until mixture cools to room temperature. Carefully add the reserved milk and egg mixture and mix until combined. Cover and refrigerate immediately.
In a medium saucepot, bring the milk and vanilla bean to a boil. In a separate bowl, whisk the eggs, sugar, and cornstarch until frothy. Temper the egg mixture with a little bit of the hot milk and return the mixture to the pot and bring to a boil over high heat. Reduce heat and simmer for 7 minutes or until mixture thickens. Transfer to a mixing bowl, cover, and refrigerate.
In a bowl of a KitchenAid fitted with the paddle attachment, combine the butter cream and praline paste until mixture becomes light and fluffy. Add the reserved praline paste and continue to mix over medium speed until just combined. Whip the heavy cream to medium/hard peaks and gently fold into the pastry cream. Transfer to a pastry bag fitted with a star tip.
Pipe the filling onto the bottom ring of the pate a choux and top with remaining pate a choux half. Sprinkle with powdered sugar and serve immediately.