Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with butter or non-stick spray. Line the bottom with parchment paper.

In a medium saucepan over low heat, melt the butter with the brown sugar for 2-3 minutes until smooth. Add cinnamon and salt, stir, and pour into the prepared cake pan.

In a separate bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon. In another bowl, cream together the butter and sugar until fluffy. Beat in eggs and vanilla.

Gradually add the dry mixture to the wet mixture, alternating with sour cream and milk. Mix until just combined.

Pour the batter over the caramel and apples and smooth the top. Bake for 40-45 minutes until a toothpick comes out clean.

Let the cake cool in the pan for 10-15 minutes, then invert onto a serving plate and serve warm or at room temperature.