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1
Place 1 1/4 cups flour, 1/4 cup sugar, 1 tsp vanilla extract and a pinch of salt in a mixing bowl. Add the egg yolk, 6 tbsp butter and 1-2 tbsp cold water. Knead with the dough hook of a mixer, then with the hands until smooth. Wrap in foil and refrigerate for 30 mins.
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2
Preheat the oven to 425u00b0F. Place a heatproof bowl of water in the bottom on the oven. Place 5 tbsp butter, 1 cup water and a pinch of salt in a medium saucepan on medium heat. Bring to a boil. Add 1 1/3 cups flour all at once and stir with a wooden spoon until you have a ball of dough that leaves the sides of the pan clean. Transfer to a mixing bowl and beat in the eggs, one at a time, with the dough hook of a mixer, until the choux dough is shiny and smooth.
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3
Cut out a 10-inch parchment paper circle and place on a baking sheet. Spoon the choux dough into a piping bag fitted with a large star tip and pipe a circle just inside the edge of the baking paper circle. Pipe a second circle on top of the first. With the remaining dough, pipe 12 tufts on the baking sheet. Bake for 25-30 mins, until golden brown and risen, without opening the door during cooking. Remove from the oven. Cool on the pan on a wire rack. Reduce the oven temperature to 400u00b0F.
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4
Grease a 10-inch springform pan and sprinkle with flour. Roll out the chilled dough to a 10-inch circle. Place in the bottom of the springform pan, pressing the dough to the edges. Prick all over with a fork. Bake for 15 mins, until golden. Remove from the oven. Cool in the pan on a wire rack.
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5
Meanwhile, mix the rhubarb and 1/2 cup sugar and let stand for 30 mins. Mix the custard powder and 3 tbsp milk in a separate bowl. Place 2 tbsp sugar and the remaining milk in a saucepan on medium heat. Bring to a boil. Add the custard mixture and return to a boil, stirring constantly. Simmer for 1 min.
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6
Remove the pastry crust from the springform pan and place on a cake plate. Spread with a thin layer of hot custard and top with the choux pastry circle. Pour the remaining custard into the middle and smooth the top. Set aside to cool.
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7
Transfer the rhubarb to a saucepan and bring to a boil, stirring gently. Simmer for 4-5 mins, until softened but not falling apart. Drain, pouring the juice into a measuring cup. Add water to the juice to measure 1 1/4 cups. Transfer to a saucepan and bring to a boil. Stir in the cornstarch paste and return to a boil. Remove from the heat and gently stir in the rhubarb. Immediately pour over the custard and refrigerate for 1 hour 30 mins.
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8
Whip the cream and 1 tsp vanilla extract until stiff peaks from. Spoon into a piping bag fitted with a medium star tip. Cut the choux tufts in half horizontally, pipe half the cream on the bottom halves and gently press on the top halves. Pipe the remaining cream in 12 tufts around the top of the cake and gently press on the choux tufts. Dust with powdered sugar.
