Chocolate Hazelnut Saint-Honoré Cake

For the Chocolate Shortcrust Pastry

1. In a large bowl, combine almond flour, cocoa powder, salt, and sugar. Sieve to remove lumps.
2. Add the cold, cubed butter and rub into the dry ingredients with your fingertips until the mixture resembles coarse breadcrumbs.
3. Whisk the egg yolk and cold water, then add to the flour mixture. Mix until a dough forms, being careful not to overwork it.
4. Flatten the dough into a disc, wrap in plastic wrap, and chill for at least 30 minutes.
5. Preheat oven to 180°C (350°F). Roll out the dough to a 3mm thickness and line a 24cm (9.5-inch) tart ring. Prick the base with a fork.
6. Bake blind for 15-20 minutes, then remove baking beans/foil and bake for another 5-10 minutes until lightly golden. Let cool completely.

For the Choux Pastry

1. In a saucepan, bring water, butter, sugar, and salt to a rolling boil. Remove from heat.
2. Add the flour all at once and beat vigorously with a wooden spoon until the mixture forms a ball and pulls away from the sides of the pan.
3. Return to low heat for 1-2 minutes, stirring constantly, to dry out the dough.
4. Transfer the dough to a mixing bowl. Gradually beat in the eggs, one at a time, ensuring each is fully incorporated before adding the next. The dough should be smooth and glossy and fall slowly from a spoon in a 'V' shape.
5. Preheat oven to 200°C (390°F). Pipe small choux buns (about 2-3 cm) onto a baking sheet lined with parchment paper.
6. Bake for 20-25 minutes, until golden brown and puffed. Turn off the oven and leave choux buns inside with the door slightly ajar for 10 minutes to dry out. Let cool completely.

For the Chocolate Hazelnut Cream

1. Heat milk in a saucepan until simmering. In a separate bowl, whisk egg yolks, sugar, and cornstarch until pale.
2. Temper the egg mixture by gradually whisking in about half of the hot milk. Pour the egg mixture back into the saucepan with the remaining milk.
3. Cook over medium heat, stirring constantly, until the cream thickens. Do not boil vigorously.
4. Remove from heat. Add chopped dark chocolate and hazelnut paste, stirring until melted and smooth.
5. Stir in the cold butter until fully incorporated. Transfer the cream to a clean bowl, cover the surface directly with plastic wrap to prevent a skin from forming, and chill until firm.

Assembly

1. Once chilled, whisk the chocolate hazelnut cream until smooth. Fill the cooled choux buns with the cream using a piping bag.
2. Arrange the filled choux buns around the edge of the chocolate shortcrust pastry base.
3. For the glaze: Heat heavy cream until just simmering. Remove from heat and pour over chopped dark chocolate and corn syrup (if using).
4. Let stand for 5 minutes, then stir until smooth and shiny. Allow the glaze to cool slightly until it thickens but is still pourable.
5. Carefully pour or spread a thin layer of the dark chocolate glaze over the entire tart, including over the choux buns. Ensure an even, glossy finish.
6. Garnish immediately with chopped toasted hazelnuts before the glaze sets.
7. Refrigerate for at least 1 hour to set the glaze and allow flavors to meld. Serve chilled.