Heat the oven to 350°F. Spread the hazelnuts on a baking sheet and toast for about 8–10 minutes, until fragrant. Let them cool, rub away as much of the loose skin as you easily can, and roughly chop them. Don't worry about getting every bit of skin off. Grease a 9-inch springform pan and line the bottom with parchment paper.

Peel and core the apples. Chop one apple into small pieces. Cut the second apple into thin slices and set those aside for the top of the cake.

In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt.

In a large bowl, beat the softened butter, brown sugar, and granulated sugar until light and creamy. Beat in the eggs one at a time, followed by the vanilla.

Mix in the sour cream. Add the dry ingredients and milk, mixing just until combined. Don't overmix. Fold in the chopped apple and about 3/4 cup of the toasted hazelnuts.

Spread the batter into the prepared pan. Arrange the apple slices over the top in a loose circular pattern. Scatter the remaining hazelnuts around the apples and sprinkle the top with 1 Tablespoon sugar.

Bake for 45–55 minutes, until the top is golden and a tester inserted into the center comes out clean or with a few moist crumbs. If the top is browning too quickly toward the end, loosely cover it with foil.

Let the cake cool in the pan for about 15 minutes before removing the springform ring. Cool until just warm or completely cool.

Just before serving, give it a generous dusting of confectioners' sugar.