Apple Crumble Coffee Cake

Instructions

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1. 1 Gather all ingredients.
2. 2 Preheat the oven to 350 degrees F (175 degrees C) with an rack in the center position. Generously butter a 9x12-inch baking dish with 2 teaspoons butter.
3. 3 Whisk flour, baking powder, baking soda, and ½ teaspoon sea salt together in a bowl; set dry ingredients aside.
4. 4 Combine walnuts, ⅓ cup brown sugar, ⅓ cup white sugar, 3 tablespoons melted butter, cinnamon, and ¼ teaspoon salt in a bowl until walnuts and sugars are thoroughly coated with butter; set crumble mixture aside.
5. 5 Beat 1 cup white sugar and ½ cup butter together in a separate bowl with a spatula until well blended.
6. 6 Whisk in 1 egg until mixture is smooth, 2 to 3 minutes.
7. 7 Whisk in second egg until thoroughly incorporated. Add yogurt and vanilla; whisk together.
8. 8 Add flour mixture to yogurt mixture; whisk just until flour disappears. Do not overmix.
9. 9 Remove and discard cores from apples. Cut apples into ⅛- to ¼-inch slices. Stack a few slices; cut down the center, lengthwise, then dice, crosswise, into cubes.
10. 10 Fold apples into cake batter with a spatula until just combined.
11. 11 Spread ½ the batter into the prepared baking dish; evenly scatter ½ crumble mixture over top.
12. 12 Dollop remaining ½ batter on top crumble mixture; spread carefully to evenly distribute, trying not to disturb the crumbs.
13. 13 Top with remaining ½ crumble mixture; press crumbs into the batter very gently.
14. 14 Bake in the center of the preheated oven until a toothpick or bamboo skewer inserted into the center comes out clean, about 40 minutes.
15. 15 Cool to room temperature before slicing and serving, about 30 minutes.