Preheat the oven to 175°C (350°F). Grease a 9 inch round cake pan and line the bottom with parchment paper. Leave the pan ready while you prepare the batter.

Add the all purpose flour, baking powder, ground cinnamon, and salt to a medium bowl. Whisk until evenly mixed, then set the bowl aside.

Place the softened butter and granulated sugar in a large mixing bowl. Beat for 2 to 3 minutes until pale and creamy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until the mixture looks smooth.

Add the sour cream and mix until incorporated. Gradually add the dry ingredients, mixing gently after each addition. Stop mixing as soon as no dry flour remains. Avoid overmixing, since that can make the cake dense

Fold the diced apples and chopped walnuts into the batter using a spatula. Spread the batter evenly in the prepared cake pan. Make sure the apple pieces are distributed throughout the batter rather than sitting in one area.

Bake for 38 to 45 minutes, or until the center is set and a toothpick inserted into the middle comes out clean or shows a few moist crumbs. If using a thermometer, the center should reach about 200°F to 205°F (93°C to 96°C). Cool the cake in the pan for 10 minutes, then transfer it to a wire rack.

Melt the butter in a small saucepan over medium heat. Add the brown sugar and heavy cream, then stir until the sugar dissolves and the mixture becomes smooth. Let it gently simmer for 3 to 5 minutes, stirring frequently. Remove the saucepan from the heat and stir in the vanilla extract and salt.

Let the toffee sauce cool for a few minutes so it thickens slightly but remains pourable. Place the cooled cake on a serving plate and spoon the warm sauce over the top, allowing some sauce to drip down the sides. Slice and serve while the sauce is warm and glossy.