Preheat oven to 350 degrees F. Spread walnuts on a baking sheet
or in a shallow pan. Bake, stirring once or twice, until lightly browned and fragrant, 10-12 minutes. Let cool slightly, then chop coarsely. Leave oven set at 350 degrees F.
Butter bottom and sides of a 12-cup Bundt pan
or a 10-inch diameter tube pan, or spray with vegetable oil
spray. (If using a baking pan
with a dark finish, reduce oven temperature to 325 degrees F.)
In a small bowl, whisk together flour
, baking powder, and salt.
In a large bowl, beat butter and granulated sugar with an electric mixer on medium-high speed until light and fluffy, about 2 minutes. Add eggs, one at a time, beating well after each addition; beat in vanilla.
With mixer on low speed, add flour mixture in 3 portions, alternating with buttermilk. Stir in 1-1/2 cups of the toasted walnuts. Scrape batter into pan, smoothing the top. Bake until a skewer or knife inserted into the center comes out clean, about 50 minutes. Transfer to a wire rack to cool completely in the pan. Invert onto a serving plate.
In a medium bowl, combine powdered sugar and water; whisk until smooth. Drizzle mixture over cake and sprinkle with the remaining 2 tablespoons walnuts.