Steps
- Mix 100g of softened butter, 120g of light brown sugar, 80g of all-purpose flour, and 40g of finely ground hazelnuts. Roll between parchment to ¹⁄₁₆in / 2mm thick and freeze.
- Bring 100ml of milk, 150ml of water, 1 teaspoon of superfine sugar, ½ teaspoon of salt, and 120g of cubed butter to a boil. Remove from the heat and beat in 190g of all-purpose flour.
- Stir over low heat for 1 minute, until the dough pulls away. Beat in a mixer for 30 seconds to cool, then gradually add 5 beaten eggs, stopping when the smooth paste drops slowly from a spoon.
- Preheat the oven to 350°F / 180°C. Pipe 3in / 8cm rings onto lined baking sheets, leaving the centers open.
- Cut matching craquelin rings with a 3in / 8cm outer cutter and 2in / 5cm inner cutter. Set on the choux and bake for 30-35 minutes, until golden, light, and dry; cool completely.
- Warm 300ml of milk until steaming. Whisk 5 egg yolks, 60g of superfine sugar, and 30g of cornstarch, then slowly whisk in the milk.
- Return to medium heat and whisk until thick. Stir in 150g of hazelnut butter and 1 large pinch of salt, then spread on a plate and cool completely.
- Beat 225g of softened butter until creamy, then beat in the cold hazelnut custard in two additions until smooth. Split the choux rings, pipe in the mousseline, replace the tops, and dust with icing sugar.