How to Make It
Preheat oven to 350°F. Lightly grease a 9x13 baking dish with butter or nonstick spray.
Make the base: Combine eggs, milk, whiskey and vanilla in a blender and blend until evenly combined, 1 minute. Add sugar, crème fraîche, flour, and salt and blend until smooth. Pour the custard into the prepared baking dish.
Make the cake mix: Whisk flour, pecans, sugar, cocoa powder, baking powder, baking soda, and salt in a medium bowl and sprinkle evenly over the custard. Using the largest holes on a box grater, grate the butter evenly over the surface of the cake.
Bake until top is set around the edges but still wobbly in the center, 30 to 35 minutes. Cool 30 minutes before serving.