Preheat your oven to 350 degrees F, make sure a rack is in the center of the oven and grease 2 9" round cake pans and line with parchment round.
Place the 18 tablespoons butter, granulated sugar and brown sugar in an electric stand mixer fitted with the paddle attachment and mix on medium until creamy and well blended, about 4 minutes. Add the eggs one at a time mixing well between each. Add in the sweet potato and mix for 1 minute more. Add the water and vanilla and mix on low for 1 minute.
In a medium bowl whisk together the flour, cinnamon, baking powder, baking soda, sea salt, allspice, nutmeg, ginger and cloves. Add to the sweet potato mixture and mix on low until just combined. Pour the batter into the prepared pans, smoothing the top and gently tapping on the counter several times to release any trapped bubbles.
Bake in the center of the oven for 40-45 minutes or until a cake tester comes out clean. Let cool for 15 minutes in the pan, remove from the pan and finish cooling on a cooling rack. While the cake is baking combine the sugar and water in a small saucepan over medium-low heat stirring until the sugar has melted. Whisk in the bourbon, cool slightly. Once the cake has cooled, place plastic wrap in the pan and place the cake back in the pan. Pour the bourbon syrup over the cakes. Cakes can be refrigerated overnight before frosting.
To prepare the buttercream, place the 2 sicks butter in a medium saucepan and melt over medium heat. Once melted, crank up the heat to medium high. Stand by, stirring and watching. Small golden bits will start to settle on the bottom of the pan and it will start to smell delicious and nutty. Once this happens, take it off the heat and pour into a small, heat-safe bowl to cool completely.
In an electric stand mixer fitted with the paddle attachment add the cooled brown butter, softened butter and mix on low until combined. Add in the powdered sugar and mix on low adding cream as needed until fully blended and smooth. Frost the cake leaving the sides naked or lightly frosted. Dust with nutmeg.
adapted from https://www.displacedhousewife.com/pumpkin-bourbon-cake-with-brown-butter-bourbon-buttercream/.