Preheat your oven to 325°F. Prepare an 8-inch springform pan by greasing it and lining the bottom with parchment paper. Place the pan on a baking sheet for extra protection from any possible spills.

In a large mixing bowl, cream together the softened butter and brown sugar using an electric mixer or stand mixer. Beat in the eggs one at a time, making sure to incorporate each egg fully before adding the next.

In a separate bowl, whisk together the hazelnut meal, all-purpose flour, baking powder, salt, and ground nutmeg. Gradually add this dry mixture to the wet ingredients, mixing until fully combined. Gently fold in the shredded apples.

Pour the batter into the prepared pan and level the top. Sprinkle with brown sugar and hazelnut meal for the topping.

Bake the cake in the preheated oven for 60 to 80 minutes, or until the top is golden, it begins to pull away from the sides, and a toothpick or cake tester comes out clean.

Let the cake cool in the pan for 5 minutes, then run a knife around the edge to loosen the cake from the pan. Remove the outer ring and let the cake cool completely or enjoy slightly warm.

For the toffee sauce, combine the butter, brown sugar, and heavy cream in a microwave-safe bowl. Heat on high for 30 seconds, then stir. Continue heating in 15-20 second increments, stirring between each, until the butter has melted and the mixture is smooth. Stir in vanilla extract. Serve the sauce warm over the cooled cake.