Make the streusel:
Put the walnuts, brown sugar, and cinnamon in a food processor. Process with short bursts until the walnuts are chopped into small pieces.
Make the cake:
In a medium bowl, whisk the flour, baking powder, salt, and orange zest until well blended With an electric mixer (I use the paddle attachment on my stand mixer), beat the butter on medium speed until smooth, about 1 minute. Sprinkle in the sugar and beat well until slightly fluffy, about 2 min., scraping the bowl well. Add the eggs one at a time, mixing well after each addition. Beat in the vanilla. With the mixer on low, add about a third of the flour mixture and mix until almost combined; add half of the buttermilk and mix again until just combined. Rrepeat with another third of the flour, the last half of the buttermilk, and then the remaining flour, mixing each time until just combined. Stir in the dried cranberries with a rubber spatula.
With the spatula, spread half of the batter into the loaf pan and sprinkle evenly with the walnut streusel. Spread on the remaining batter. Bake until the loaf is golden brown and a skewer inserted in the middle comes out clean, 60 to 75 min. Let cool in the pan on a rack for 10 min., turn out of the pan, and let cool completely on the rack.
Make the icing:
In a small bowl, mix the confectioners’ sugar, cream, and cinnamon with a small spoon until there are no lumps. The icing should drizzle smoothly in a thin line from the tip of the spoon; you may need to add up to another 1 tsp. cream. Drizzle the icing over the cake in thin parallel lines and let set for about 15 minutes before serving. Slice gently with a serrated knife.