Prep Pan and Apples: Preheat your oven to 180°C (350°F) and line a 9x9-inch baking pan with parchment paper. Peel and chop your apples into small cubes.
Mix Wet Ingredients: In a large mixing bowl, whisk together the eggs, cooled melted butter, cinnamon, yogurt, granulated sugar, milk, kosher salt, and vanilla extract until well combined.
Add Dry Ingredients: Add the flour, baking powder, and baking soda, and mix with a whisk until almost combined.
Fold in Apples: Add the chopped apples and mix gently until just combined with no visible flour lumps. Do not overmix.
Make the Swirl and Bake
Make the Cinnamon Topping: In a small mixing bowl, combine melted butter, cinnamon, and sugar, mixing with a spoon until well combined.
Assemble and Swirl: Pour the cake batter into your prepared 9x9-inch pan and smooth it out with a spatula. Spoon the cinnamon topping over the cake and create swirls using a butter knife.
Bake: Bake in a preheated 180°C (350°F) oven for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean.
Glaze and Serve
Make the Glaze: In a small mixing bowl, combine the powdered sugar, milk, and vanilla, stirring until smooth and silky.
Finish: Drizzle the glaze over the cake, slice, and serve warm next to a cup of coffee or hot tea!
Notes
Room-temperature ingredients: Always use room-temperature eggs, milk, and yogurt so the batter emulsifies properly and the cake bakes evenly.
Choosing your apples: Opt for firm baking apples like Granny Smith, Honeycrisp, or Braeburn that hold their shape well and provide a great balance of sweetness and tartness.
Measuring flour correctly: Fluff your flour with a spoon, gently spoon it into your measuring cup, and level it off with a knife to prevent a heavy or dense texture.
Gentle mixing: Fold the flour and apples into the batter gently just until combined to avoid developing gluten, ensuring a soft and tender crumb.
Cooling before slicing: Let the cake cool completely to room temperature before slicing so it holds its structure and does not fall apart.
Storage and shelf life: Store the cake in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 5 days.
Freezing instructions: Freeze the cooled and glazed cake by wrapping it tightly in plastic wrap and aluminum foil for up to 3 months. Thaw overnight in the refrigerator before serving.