Preheat oven to 350°F (175°C).

Combine pecans, brown sugar, melted butter, sorghum syrup, and water in a bowl.

Spread pecan mixture evenly in a greased 10-inch cast-iron skillet or 9-inch cake pan.

In a separate bowl, beat granulated sugar and softened butter until smooth.

Add eggs one at a time, beating well after each addition.

In another bowl, whisk together flour, pumpkin pie spice, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.

Stir in mashed sweet potatoes, buttermilk, and vanilla extract until smooth.

Pour the cake batter over the pecan mixture in the skillet.

Bake for 40-45 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the skillet for 10 minutes.

Run a knife around the edge of the cake to loosen.

Invert the cake onto a serving plate.

Spoon any remaining topping from the skillet over the cake.