Preheat oven to 350 degrees F. Butter and flour two 8x3-inch cake pans. Line the bottoms with a round of parchment. Set aside.
In a medium bowl, whisk together the flour, baking powder and salt. In the bowl of a stand-up mixer add the butter and sugar; cream together until light and fluffy, about 2 minutes. Crack in one egg at a time, adding the next egg only when the one before it has combined. Next, add in the vanilla extract and the reserved chestnut paste. Turn the mixer down to low speed and add the buttermilk and flour mixture in a few batches, alternating between the two.
Divide the cake batter amongst pans, smoothing out the top with a spatula (the batter will be a little thick so it’ll need some help smoothing out). Transfer to the oven and bake for 35 to 40 minutes, or until a a skewer inserted into the center comes out clean. Cool the cakes in their pan for 10 minutes before running a knife along the side of the cakes and inverting them onto a wire rack to cool completely.
To assemble: place the first layer of cake on a cake board or a cake stand. Add about 1/4 cup of frosting the top of the cake layer and smooth it out, pushing it out to the sides. Add the second layer on top and then add a second 1/4 cup to the top of that. (These measurements can be eyeballed.) Smooth out the top and add a nice thin layer all the way around the sides of the cake. Transfer to the freezer to chill for about 5 to 7 minutes. This is the crumb coat. Add a second even layer on the outside. I used a small star-tip to create an over under, over under effect. I topped it with sliced cranberries and rosemary that were dusted with powdered sugar. Slice and serve!