Bring a pot of salted water to a boil. In a mixing bowl, combine the butternut puree with the mascarpone cheese and season, to taste, with salt and white pepper. In the center of 6 pasta rectangles, place 2 tablespoons of the butternut puree mixture. Sprinkle with the chopped pecans. Slightly wet the edges of the pasta with a little cool water. Lay the other 6 pasta rectangles over the filled pasta rectangles. Gently seal each ravioli by pressing around all edges with the tines of a fork. Place the ravioli in the boiling water and cook for 3 to 5 minutes, or until the pasta is cooked al dente. Remove from the water and drain. Season the ravioli with salt and pepper.

Heat a saute pan over medium-high heat and, when hot, add the butter and let it melt in 1 spot. (Do not move the pan.) When the butter has begun to brown around the edges, pick up the saute pan and swirl to keep the melted butter from burning and to melt the remaining butter. Add the sage leaves and reduce the heat to medium. Continue to cook until the leaves are crispy, 1 to 2 minutes. Season with salt and pepper. To serve as an appetizer, lay 1 ravioli in a shallow bowl and spoon the brown butter over the top. Garnish with the crispy sage leaves. Sprinkle with Parmigiano-Reggiano and serve.