Blend all crepe batter ingredients in blender for 25 seconds, until smooth. Let sit for 15 minutes.
When ready to make crepes, heat a 10 inch non-stick skillet over medium heat. Melt a dab of butter in pan, and pour in about 3 tablespoons batter, tilting the pan so that the batter covers the surface entirely. Let cook for 2-3 minutes, until you see bubbles and the edges flare a bit. Then using a long spatula, flip over and cook for 2 minutes on the other side.
Set on a wire rack to cool.
When you're ready to assemble the cake, add all cream ingredients to a large bowl or the bowl of a stand mixer. Mix on medium until all combined and smooth. Then fold in hazelnuts.
Place one crepe on a cakestand or plate, then spread a thin layer of pastry cream over the top. Stack another crepe, then another thin layer of pastry cream. Repeat until you reach the top of the cake, and put any leftover cream on top in the middle, with a mound of toasted hazelnuts. Refrigerate for 15 minutes if possible, as it slices easiest this way.
Cake will last, covered, in refrigerator for up to 3 days.
