Preheat oven to 350°F (175°C). Grease and line the bottom of a 9-inch round cake pan with parchment paper.

Prepare the topping: In the prepared cake pan, pour the 1/2 cup of melted butter evenly over the bottom. Sprinkle the brown sugar evenly over the butter.

Arrange the sliced nectarines decoratively over the sugar layer, overlapping slightly in a circular pattern. Sprinkle the slivered almonds over the fruit.

Prepare the batter: In a large bowl, whisk together the all-purpose flour, granulated sugar, almond flour, baking powder, and salt.

In a separate bowl, cream the softened butter until light and fluffy. Beat in the eggs one at a time, followed by the vanilla extract. Mix in the milk and Amaretto (if using).

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.

Gently spoon the batter over the fruit and almond topping, spreading it carefully to cover the entire surface without disturbing the fruit pattern.

Bake for 45 to 50 minutes, or until a wooden skewer inserted into the center comes out clean. The cake edges should be golden brown.

Let the cake cool in the pan for exactly 10 minutes. Place a serving plate upside down over the cake pan, and carefully invert both together. Lift the pan off and gently peel away the parchment paper. Allow to cool slightly before slicing.