Preheat oven to 375 degrees F. Butter a jelly roll pan and line with parchment. Set aside.
Take the carrots and chop into large pieces, then mince in a food processor until you have a fine texture. Drain pineapple and finely chop walnuts. Set aside. In a large mixing bowl add flour, baking soda, baking powder and spices together. In a separate bowl mix buttermilk, molasses, eggs, vegetable oil and dark brown sugar together.
Now combine wet and dry ingredients together to make a batter then fold in the carrots, pineapple and walnuts. Pour into the prepared pan and bake in the oven for 25-30 minutes until cake is set and springs back when gently pressed in the middle. Remove the pan from the oven and allow cake to cool on a rack while you prepare the frosting and cointreau carrots (recipes below). Once the cake has cooled, cut into three rectangles by cutting the cake lengthwise twice. Stack the cake up into three tiers with cream cheese frosting in between each layer. Frost the outside of the entire finished cake - smoothing off the edges and corners (an offset spatula works well). Top with Cointreau carrot rounds and drizzle a little of the syrup on top as well.
Using a kitchen mixer, combine cream cheese and butter until blended and you have a smooth, light texture. Add the vanilla, lemon zest and powdered sugar and beat until combined. Continue to beat until smooth and glossy - about 7 minutes.
Slice carrot up into rounds. Add carrot, butter, sugar, cointreau and salt to a small saucepan over medium heat. Bring to a simmer and reduce until syrupy - about 5 minutes.