Preheat the oven to 325°F and grease and flour two 8 or 9 inch cake pans. Set aside.
In the bowl of a standmixer fitted with a whisk attachment, beat the softened butter and oil together until they become homogenous, about 3 minutes. This will not work if your butter is not completely room temperature!
Slowly add the dark brown sugar sugar to the oil and butter, beating on high until the mixture is light and fluffy and lightens in color, about 3 minutes.
Add the molasses and mix until combined. Add the eggs in one at time, scraping down the bowl in between each addition. Add the vanilla extract. Beat the mixture on high for 3 minutes more to incorporate as much air as possible.
In separate bowl sift together the flour, salt, baking powder, and spices. Alternate adding in the dry ingredients and the buttermilk to the butter mixture in 4 batches, starting with the buttermilk and ending with the dry ingredients. It’s okay if the batter still has a few lumps in it–you don’t want to over mix it.
Split the batter evenly into the prepared pans and bake for 30 minutes or until the cake is a lightly springs back when pressed and a knife inserted into the middle comes out with just a few moist crumbs attached. Take out of the oven and transfer to a cooling rack. Let the cakes cool completely.
Once the cakes have cooled completely, make the frosting. Place the white chocolate into a large microwave-safe bowl. Microwave in 15 second increments, using a silicone spatula to stir between each, until the white chocolate is fully melted. Set aside to cool slightly–you still want it to warm and fully melted, but not so hot that it will melt the butter.
In a stand mixer fitted with a paddle attachment, beat the butter, confectioners’ sugar, vanilla, and salt on low speed until combined. With the mixer still running, slowly stream in the melted white chocolate. Once incorporated, increase the speed to medium-high and beat until light and fluffy, about 1 minute, scraping down the sides and bottom of the bowl as needed.
Place the first layer of cake right side up onto a cake stand or plate. Spread about 1/3 of the frosting on top, using an offset spatula to spread a thick layer evenly to the edges of the cake. Top with the second layer and use the remaining frosting to frost the top and sides of the cake. If desired, top with gingerbread cookies or rosemary trees! Refrigerate until ready to serve!
Notes
I prefer to use dark brown sugar because it gives the cake a deeper flavor, but light brown sugar works just as well!