Prepare: Preheat the oven to 350º F/180º C. Prepare two 8" round cake pans by coating with butter and dusting with flour.
Wet Ingredients: Cream together the butter and brown sugar until thick and fluffy. Beat in the vanilla, molasses, and eggs.
Dry Ingredients: In a separate bowl, whisk together the dry ingredients. In a third bowl, stir together the water and buttermilk. Add ⅓ of the dry ingredients to the wet ingredients, beat to combine, then add ⅓ of the buttermilk mixture and beat to combine. Continue alternating until everything is combined and smooth.
Bake: Pour half the batter into each cake pan and bake for 40 minutes or until a skewer inserted into the middle of the cakes comes out clean. Remove from the oven and let cool in the pans for about 15 minutes, then remove from the pans and let cool fully.
Make the Caramel: While the cakes bake, add the butter, brown sugar, vanilla, milk, and salt to a small saucepan. Heat uncovered over medium heat, stirring occasionally, until the mixture comes to a gentle boil. Boil for 1-2 minutes, stirring constantly, until the mixture thickens slightly. Remove from the heat and let cool to room temperature.
Make the Frosting: Using a stand mixer, beat the butter until smooth. Add the penuche caramel and beat until fully combined. Add the powdered sugar ½ cup at a time, mixing fully after each addition.
Assemble the Cake: Stack the cooled cakes with a layer of penuche frosting in between. Add a dollop of frosting on top, then use an offset spatula or butter knife to smooth it down the sides. Use a straight edge to neaten the top and sides of the cake. If desired, decorate with chopped nuts, berries, and colored frosting. I used a small round tip for the berries, a small leaf tip for the leaves, and an offset spatula for the branches. Refrigerate until cold, then slice and serve.