Preheat the oven to 350°F (175°C). Grease and flour your bundt pan thoroughly.
In a large mixing bowl, beat softened butter, brown sugar, and granulated sugar until light and fluffy.
Beat in the eggs one at a time, then mix in vanilla extract.
In a separate bowl, whisk together flour, baking powder, and cinnamon.
Gradually add the dry mixture to the wet ingredients, alternating with diced apples. Stir just until combined.
Pour the batter evenly into the prepared bundt pan. Bake for 50–60 minutes or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then carefully transfer to a wire rack to cool completely.
In a small bowl, whisk together powdered sugar, brown sugar, and milk until smooth. Drizzle over the cooled cake before serving.
