Preheat the oven to 350 F with the rack positioned in the bottom third of the oven. Butter a 9" x 5" loaf pan.
To make the streusel: Stir together the flour, brown sugar, cinnamon and salt in a small mixing bowl. Using a pastry blender or 2 forks, cut in the butter until it creates pea-sized crumbs. Set aside.
To make the bread: In a medium sized mixing bowl, whisk together the melted butter, sugar, pumpkin, milk, eggs and vanilla until combined.
In another mixing bowl, sift together the flour, cinnamon, baking soda, salt, ginger, nutmeg and baking powder. Add the flour mixture to the pumpkin mixture and mix with a rubber spatula just until combined. Do not over mix. Gently fold in the chopped toasted walnuts. Spoon half the batter into the prepared loaf pan. Spread the streusel over the batter and cover with the remaining batter. Place the walnut halves over the surface of the bread.
Bake until the top is golden brown and a toothpick inserted into the center comes out clean, 65 to 70 minutes. Transfer the pan to a wire cooling rack and let cool for 15 minutes. Using a thin knife or spatula, release the bread from the pan and let cool to room temperature.
Makes one loaf.