Ingredients

For the cake:
• 2 cups (250g) all-purpose flour
• 1½ tsp baking powder
• ½ tsp baking soda
• ½ tsp salt
• 1½ tsp ground cinnamon
• ¾ cup (1½ sticks / 170g) unsalted butter, softened
• ¾ cup (150g) granulated sugar
• 2 large eggs
• 1 tsp vanilla extract
• ¾ cup (180ml) sour cream or plain Greek yogurt
• 2 cups peeled & finely diced apples (Honeycrisp or Granny Smith)

For the cinnamon swirl (optional but recommended):
• ¼ cup (50g) packed brown sugar
• 1 tsp cinnamon

For the crumb topping:
• ¼ cup (50g) granulated sugar
• ¼ cup (55g) packed brown sugar
• 1 tsp cinnamon
• 3 tbsp all-purpose flour
• 3 tbsp cold unsalted butter, cubed

Instructions

Preheat oven to 350°F (175°C). Grease a 9″ round or square baking pan; line bottom with parchment.
Crumb topping: Mix sugars, cinnamon, and flour. Cut in cold butter until crumbly. Set aside.
Cake: Whisk flour, baking powder, baking soda, salt, and cinnamon.
In another bowl, beat butter and sugar until light. Add eggs one at a time, then vanilla.
Alternate adding dry ingredients and sour cream to wet mixture, beginning and ending with flour. Fold in apples.
Swirl (optional): Spread half the batter into pan. Sprinkle with brown sugar–cinnamon mix. Top with remaining batter.
Sprinkle crumb topping evenly over surface.
Bake 40–45 minutes until golden and a toothpick comes out clean (tent with foil if topping browns too quickly).
Cool 15 minutes before slicing. Best served warm!