Preheat the oven to 350°F (175°C).
Grease and flour two 9-inch round cake pans.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.
Add eggs one at a time, ensuring each is fully incorporated before adding the next.
Mix in lemon zest, elderflower cordial, and Irish whiskey, blending well.
In another bowl, whisk together the flour, baking powder, and salt.
Alternately add the dry ingredients and buttermilk to the wet mixture, beginning and ending with the flour mixture.
Divide the batter evenly between the prepared cake pans and smooth the tops.
Bake in the preheated oven for 25-30 minutes or until a toothpick comes out clean.
Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.
While the cake is cooling, prepare the frosting by combining powdered sugar, lemon juice, and elderflower cordial until smooth.
Once the cakes are completely cooled, frost the top of one layer, stack the other layer on top, and frost the top and sides.
Decorate with additional lemon zest or edible flowers if desired, then slice and serve.