For the sauce: Heat the butter over medium heat in a small saucepan until the milk solids begin to brown and butter smells toasted, about 6 minutes; remove from heat. Pour 6 tablespoons of the butter into a bowl and set aside. To the remaining 2 tablespoons browned butter left in the pan, whisk in the sugar, cornstarch and salt until it forms a thick paste. Add the milk, bourbon and vanilla, whisking until smooth. Return the saucepan to medium heat and cook, stirring often, until the mixture comes to a boil and thickens to the consistency of loose pudding, about 3 minutes. Transfer the sauce to a serving bowl; keep warm.
For the pancake: Toast the pecans in a large skillet over medium heat until they smell nutty, about 4 minutes. Transfer the pecans to a cutting board to cool for 5 minutes. Roughly chop enough pecans to measure 1/2 cup and reserve for garnish. Finely chop the remaining pecans (you should have about 1 cup) and reserve for the pancake batter.
Whisk the all-purpose and buckwheat flours, the cornstarch, confectioners' sugar, baking powder, baking soda and salt together in a large bowl. Add the 1 cup finely chopped pecans and whisk again to evenly coat. Whisk the reserved 6 tablespoons browned butter, the milk, vanilla and eggs together in another bowl. Add the milk mixture to the flour mixture and stir until ingredients are just incorporated and form a thick batter (it's okay if there are some lumps). Let the batter rest for 5 minutes.
Heat a 12-inch nonstick skillet over medium heat for 5 minutes. Pour all the batter into the skillet, smoothing the top. Cook until bubbles break on the surface of the pancake, and the underside is golden brown, about 6 minutes (reduce heat if pancake is browning too quickly). To flip the pancake neatly, use a spatula to lift the edge of one side of the pancake and carefully slide it, batter side up, onto a large plate. Invert the skillet over the plate then use kitchen towels or oven mitts to carefully invert the plate and skillet together, allowing the pancake to fall back into the pan, cooked side up. Return the skillet to the heat and cook until the second side is golden brown and the pancake is completely cooked through in the middle, about 6 minutes more. Transfer the pancake to a rack and let cool for 5 minutes.
Using a serrated knife, cut the pancake into 8 wedges, placing 2 wedges on each plate. Dust heavily with confectioners' sugar, drizzle with the brown butter sauce and top with reserved roughly chopped pecans.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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