In a small bowl, soak raisins in cognac for 30 minutes. Then drain dry fruit with a colander and set aside. 

To make the brioche dough, using the hook attachment of an electric mixer or the flat beater of the stand mixer, mix sifted flour, salt, sugar, and eggs at medium speed for about 5 minutes.

In a separate small bowl, warm up the milk (it should be lukewarm) and dissolve the instant yeast, then add the milk/yeast mixture to the main preparation. Using the dough hook of the electric or stand mixer, continue to mix for 15 minutes longer.

Keep the mixer running at medium speed, gradually add softened butter, and cut it into pieces. Once the butter is incorporated, mix the preparation for 10 to 15 minutes longer until the dough becomes soft and silky. Transfer the dough to another bowl, cover with a kitchen towel, and let it rest for 2 hours.

Finally, add drained raisins and chopped nuts. Mix the dough with your hands, cover the bowl with a food film, and refrigerate overnight.

On the next day, butter cavities of the silicone Kouglof mold and place the dough, filling up to ¾. Cover the mold with the kitchen towel and let the dough rest in a warm place for 2 hours.

Preheat oven to 390 F/200 C and bake Kouglof for 10 minutes. Then decrease the oven temperature to 340 F/170 C and bake for 15 to 20 minutes longer. Take brioches out of the oven, let them cool, and remove them from molds.

To make the syrup, mix all the ingredients and bring to a boil. Pour the ready syrup into a deep bowl and dip each brioche in a hot liquid for a few seconds. Then place Kouglof on a wire rack, let the syrup drip off, and refrigerate for 15 minutes.

To make the hazelnut butter, place the butter in a saucepan and heat until it gets browned. Drain the butter into a deep bowl and dip each brioche in hot butter for a few seconds. Once again, place brioches on a rack, let the butter drip off, and refrigerate for 2 minutes. Cover each brioche with granulated white sugar, shake off excess, and transfer to a serving plate.