Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with 2 tablespoons of softened butter.

Cream together 3/4 cup of softened butter and 3/4 cup of granulated sugar until light and fluffy.

Add in 1/4 cup of vegetable oil and blend well until fully incorporated.

Whisk together the dry ingredients in a separate bowl: all-purpose flour, baking powder, baking soda, salt, allspice, ginger, and chopped rosemary.

In another bowl, combine the wet ingredients: eggs, buttermilk, molasses, and vanilla extract, whisking until uniformly combined.

Gradually add the dry ingredients to the creamed mixture, alternating with the wet ingredients, mixing just until combined.

Peel and slice sweet potatoes thinly and arrange them evenly in the prepared cake pan.

Pour the batter over the arranged sweet potatoes and bake for 30-35 minutes, until golden brown.

Allow the cake to cool in the pan for about 10 minutes before inverting onto a serving plate.