1. Preheat the oven to 350°F (180°C). Place the hazelnuts on a baking sheet lined with baking parchment and toast them for 8-10 minutes. Remove the nuts from the oven, place them on a clean kitchen towel and wrap them with it – allow them briefly to evaporate some moisture, then use the kitchen towel to rub off the skins of the hazelnuts. If the skins don’t come off completely that’s fine. Set aside. Have the baking sheet ready with a clean piece of baking parchment.

2. Add sugar and water to a small saucepan and heat up until the water has evaporated completely and the sugar begins to melt. Gently caramelize the sugar until it turns an amber color. Remove from the heat, add the toasted hazelnuts and fold in. Pour the mixture on the lined baking sheet and spread a little. Let cool completely. Break the hazelnut brittle into smaller pieces and mix/blend in a strong blender/food processor until you get a fine paste. Transfer to a clean container and refrigerate until further use (max. 2 weeks).

3. Combine the milk for the crème mousseline with the hazelnut paste in a small saucepan and heat up until the paste has dissolved. Remove from heat and let cool slightly. Whisk the egg yolks, sugar, and cornstarch in a small bowl until thick and fluffy. Add to the cooled milk in the saucepan and whisk. Return to the heat and bring to a boil, stirring constantly, until you get a thick custard. Remove from heat and immediately cover with a piece of plastic wrap to prevent skin from forming. Let cool down completely, then refrigerate for at least 1 hour.

4. Place the butter for the cream in the bowl of a food processor and mix until light and fluffy. Gradually add the cooled hazelnut custard and stir in. Mix on high for a few minutes until you get a smooth and fluffy crème mousseline. Place in the fridge until needed.

5. For the choux pastry, combine milk, water, sugar, and salt in a small pot. Add butter and heat up until the butter has melted completely. Bring to a boil, then add the flour (all at once) and stir in with a wooden spoon. Mix until you get a smooth dough ball that comes off the bottom and sides. Continue until you see a white layer on the bottom of the pot. Transfer to a large bowl and let cool down a bit (about 2-3 minutes).

6. When the dough has cooled down you can stir in the eggs – one at a time. I use a hand mixer with a dough hook but you can also do the job with a wooden spoon, it just takes longer. Each egg should be completely incorporated before the next one is added. The dough will be quite compact but should look smooth and shiny. When pulling the dough hooks or wooden spoon out of the dough, it should drip/fall down in a ribbon shape. If it doesn’t do that after three eggs, gradually add some more slightly whisked egg until the consistency is right. Fill the batter into a piping bag with a large star nozzle. Pipe dough circles with a diameter of about 4.3 inches on two baking sheets lined with baking parchment. It helps to draw circles on the paper beforehand (and flip the paper) the ensure all circles are the same size. Let the dough rest for about 10-15 minutes – meanwhile, you can preheat the oven.

7. Preheat the oven to 390°F (200°C). Have some ice cubes ready. Carefully brush the dough circles with some milk and sprinkle with sliced almonds. Place the first baking sheet in the oven and place 1-2 ice cubes on the bottom of the oven to create some steam and bake the choux pastry rings for about 25-30 minutes. Do not open the oven in between to check! The choux pastry is perfect when it has risen nicely with a crispy-looking surface and golden brown color. Remove from the oven and pull the rings with the baking paper onto a cooling rack and let cool completely. Repeat the process (including the ice cubes) with the second baking sheet.

8. Cut the completely cooled choux pastry rings in half horizontally. Whisk the crème mousseline once again briefly, if necessary, then fill into a piping bag with a large star nozzle and fill the bottom halves of the pastry rings with the cream. Place the lids on top and dust with some confectioners’ sugar before serving.