How to Make It
Preheat oven to 350°.
To prepare filling, combine first 4 ingredients in a saucepan over medium-high heat; bring to a boil. Reduce heat to medium; simmer 20 minutes or until liquid almost evaporates and apple is tender, stirring occasionally. Cool to room temperature. Discard cinnamon stick.
To prepare streusel, combine walnuts and next 3 ingredients (through ground cinnamon) in a food processor. Pulse until mixture resembles coarse meal.
To prepare cake, coat a 12-cup bundt pan with baking spray. Place sugar and butter in a large bowl; beat with a mixer at medium speed until light and fluffy (about 3 minutes). Add eggs and egg yolk, 1 at a time, beating well after each addition. Scrape seeds from vanilla bean; add seeds to sugar mixture. Reserve bean for another use. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, baking soda, and salt in a bowl, stirring well with a whisk. Combine milk and yogurt in a small bowl, stirring until smooth. Add flour mixture and milk mixture alternately to sugar mixture, beginning and ending with flour mixture; beat just until combined.
Spoon one-third of batter into prepared pan; sprinkle batter with half apple mixture and half walnut mixture. Repeat layers, ending with remaining third of batter; smooth with a spatula. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool on a wire rack for 15 minutes; remove cake from pan. Cool on a wire rack. Sprinkle with powdered sugar.