Heat the milk, whisking in cocoa powder, vanilla, and sugar until combined over medium low heat for 5 minutes, whisking occasionally until combined and warm. Cool for 5 minutes.
Using a whisk or fork and bowl, or blender, combine the buckwheat flour, salt, butter, eggs, and warm cocoa milk together until smooth. Transfer to a bowl. Cover and chill the batter for at least 2 hours, or preferably overnight.
When ready to make the crepes, whisk 1/2 cup water into the batter. Place a large dinner plate or sheet pan on the stovetop next to where you will be cooking.
Set an 8-inch nonstick skillet or crepe pan over medium heat. Drop a small knob of unsalted butter into the pan, swiping around the sides with a paper towel to coat the bottom. Turn the heat down to medium low. Taking the pan off the heat, pour and swirl in 3 tablespoons but no more than 1/4 cup batter into the pan, tilt and swirl off the heat spreading the batter over itself in the middle until the batter stops moving. Place the pan back on the stovetop and cook for 1 minute and 30 seconds or when the top looks more dry and the edges have begun to release from the sides of the pan. Nudge a flexible spatula underneath and flip, cooking on the other side for 30 seconds. Place the crepes in a stack on the plate or sheet pan. Repeat until you’ve cooked all the batter.
Place the cream and California Mission Figs in a small saucepan set over medium low heat, for about 5 minutes. Tumble the chopped chocolate or chocolate feves into a medium sized bowl. Place a strainer on top of the bowl and pour the cream through the strainer onto the chocolate below. Ignore the cream and chocolate for 3 minutes. Transfer the figs from the strainer to a food processor along with the salt and hazelnuts. Stir together the cream and chocolate until smooth. Then, transfer to the food processor. Process until smooth.
Place a crepe on a cake stand or whatever surface you plan on presenting it. Select your 20 best crepes. Using an offset spatula, spread about 2 tablespoons evenly across the crepe. Top with another crepe. Continue the crepe and ganache layers until you’ve used up all the crepes. Dollop and spread the rest of the ganache on top of the cake. You can scatter chopped toasted hazelnuts on top or slices of dried mission figs. Chill for 2 hours before serving.