Apple Crumble Coffee Cake Recipe by John Mitzewich from All Recipe.

Ingredients

Cake

• 2 teaspoons unsalted butter
• 2 cups all-purpose flour
• 1 teaspoon baking powder
• ¾ teaspoon baking soda
• ½ teaspoon fine sea salt
• 1 cup white sugar
• ½ cup unsalted butter, at room temperature
• 2 large eggs
• 1 cup plain yogurt
• 1 ½ teaspoons vanilla extract
• 2 Honeycrisp apples

Crumble Mixture

• 1 ½ cups finely chopped toasted walnuts
• ⅓ cup packed light brown sugar
• ⅓ cup white sugar
• 3 tablespoons unsalted butter, melted
• 1 teaspoon ground cinnamon 
• ¼ teaspoon salt

Directions

1. Preheat oven to 350°F and prepare a 9×12 baking dish with butter.
2. Make the crumble. Combine walnuts, brown sugar, white sugar, melted butter, cinnamon and salt. Mix until the walnuts and sugar are thoroughly coated with butter.
3. Mix the dry ingredients; flour, baking powder, baking soda, and salt.
4. Mix the wet ingredients; cream, sugar, and butter. Combine well.
5. Add in 1 egg and whisk until smooth.
6. Whisk in another egg and incorporate. Add yogurt and vanilla, and mix.
7. Add the dry flour mixture and whisk until incorporated. Don’t overmix.
8. Remove the core from the apples and cut them into ⅛ to ¼-inch slices. Dice into cubes.
9. Add to the cake batter and mix until combined.
10. Spread half of the batter into the bottom of the prepared baking dish. Pour half of the crumble mixture on top and then the rest of the batter.
11. Top with the rest of the crumble mixture, pressing the crumbs into the batter gently.
12. Bake in the center rack of the preheated oven until a toothpick comes out clean. This should take about 40 minutes.
13. Let cool for about 30 minutes and serve. Pairs great with homemade vanilla ice cream.