Yield two 4x9''/10x23cm loaf pans. Make a day ahead.
In the bowl of your stand mixer fitted with the hook, mix eggs, milk, salt, sugar, flours and instant yeast. Once the dough comes together, set the speed to medium and mix for 12 to 15 minutes scraping down the bowl and the hook as it goes. The dough should become firm and somewhat stretchy. Add the cubed butter and continue mixing on medium low speed. Scrape down the sides of the bowl as it goes. Increase the mixer speed and mix for 5 minutes more. The dough should make a slap-slap sound against the sides of the mixer bowl showing the dough elasticity – it should not stick anymore. The internal temperature should now be around 75/80ºF/24/26ºC. Transfer brioche in a greased bowl or use the mixer bowl. Cover with plastic wrap and let rise for a couple hours or until it has tripled in size. Deflate dough, form a ball, cover and refrigerate overnight. The refrigeration slows down fermentation. This gluten-relaxation lessens the elasticity of the dough, making it easier to roll out.
In a food processor, blend almond paste* 50% or 65% (look up recipe) with sugar and salt until well combined. Add flour and cocoa powder; give a few pulses. Add cubed butter and mix until just combined. Roll out into a 1/4''4mm thick sheet. Freeze and cut into strips and cubes. Bake for 12 minutes at 325ºF/160ºC. Let cool and crush slightly. Use half of the streusel for the filling and remaining for the topping.
Melt all ingredients together; smooth it out and set aside. Warm up if too firm.
Bring to a boil water and sugar. Remove from heat and add flavoring. Use syrup lukewarm.
Flour work surface and the chilled brioche dough. Roll out brioche loaves into a large 1/4''/4mm thick sheet. Keep dough cool – freeze a few minutes if too soft. Spread chocolate fudge over and half of the almond chocolate streusel. Roll out into a tight log. Divide log into 2 equal portions. Cut it lengthwise and twist. Grease pans with softened butter and add a coating of cooking spray. Let babka proof; ideally at 80°F/26ºC for about 2 hours or until the edges of the dough reach the tops of the molds. Egg-wash if desired.
Bake at 325ºF/160ºC for 30 minutes. Unmold almost immediately after coming out of the oven. Prick brioche and pour in warm syrup. Cover with remaining chocolate streusel. Enjoy!