Special equipment: a nonstick muffin pan with 12 (1/2-cup) muffin cups
Preheat oven to 375°F. Generously butter muffin cups and top of pan.
Make streusel for topping:
Whisk together brown sugar, oats, flour, cinnamon, and salt in a bowl. Blend in butter with your fingertips until mixture forms small clumps.
Whisk together flour, baking powder, cinnamon and salt in a bowl. Beat together butter and sugar in a large bowl with an electric mixer at medium speed until blended, about 2 minutes. Add eggs and vanilla and beat until fluffy, about 1 minute. Reduce speed to low and add flour mixture and milk alternately in 2 batches, beginning and ending with flour mixture and mixing until just combined. Toss rhubarb with confectioners sugar and gently fold rhubarb and walnuts into cake batter. Divide batter among muffin cups, then crumble streusel evenly on top. Bake in middle of oven until tops are golden and a tester inserted in center of a cake comes out clean, 22 to 25 minutes. Cool cakes in pan on a rack 3 minutes, then loosen edges of cakes with a small sharp knife or small metal spatula. Remove cakes from pan and put on rack to cool slightly.
Adapted from Gourmet, April 2003