For the Lemon Cake:
Preheat oven to 350°F. Butter three 8-inch or two 9-inch cake pans. Line bottoms of pans with parchment paper, and brush with soft butter.
In medium bowl, whisk together flour, baking powder and salt. Set aside.
In the bowl of electric mixer fitted with the paddle attachment, cream 2 sticks butter and sugar until light and fluffy, about 2 minutes. Add oil, lemon zest, eggs, vanilla and lemon juice. Mix on medium speed until smooth, scraping as necessary.
With mixer on low speed, add one third of flour mixture, followed by half of buttermilk, beating well after each addition. Repeat, ending with flour mixture. Increase speed and beat for another 30 seconds.
Divide batter evenly among prepared pans. Bake for 25 to 30 minutes, or until toothpick inserted into center comes out clean. Cool in pans for 5 minutes, then turn out onto cooling rack to cool completely.
For the Lemon Syrup:
Add sugar and water to small saucepan. Bring to boil. Reduce heat and simmer for 2 minutes. Remove from heat, add lemon zest and juice. Cool completely.
For the Lemon Buttercream:
Add butter to bowl of stand mixer. Beat until butter is very pale and smooth. It is ready when it makes a slapping sound as it hits sides of the bowl.
Add sugar one cup at a time. Mix until it is fully incorporated before you add next cup.
When all of sugar has been added, scrape paddle and sides of bowl. Add vanilla, salt, lemon juice and heavy cream and beat on low speed for 15 seconds. Increase speed to medium-high and beat until whipped, light and perfectly smooth, about 5 minutes. Scrape bowl. The frosting will look like it is getting really soft, but it will stiffen and increase in volume.
Remove approximately 2 cups of frosting and put into 4 small bowls.
To Assemble the Cake:
If needed, even cake layers with serrated knife. Place cake on cake plate and brush cake liberally with lemon syrup. Spread layer of frosting across top. Repeat with all cake layers. Apply very thin layer of frosting to sides and top of cake to seal in crumbs. Let chill for 30 minutes.
Apply thicker layer of frosting to sides and top of cake, covering cake completely. Smooth with offset spatula. Chill for 20 minutes.
To Decorate the Cake:
Tint 4 bowls of reserved frosting with golden yellow, pink, lavender and green gel coloring. Add to piping bags.
With Wilton Tip #69, pipe green leaves along the top edge and down the side of the cake to build flowers on. With Wilton Tip #20, pipe pink roses on top of green leaves by starting with a small circle, and moving the circle outwards in a swirling motion. Repeat until you have 6 roses around the outside of cake. With Wilton Tip #21, pipe lavender flowers by holding piping bag straight over top of cake and applying pressure to pipe flowers. Using Wilton Tip #32, fill in with smaller yellow flowers, using the same process as the lavender ones. Continue adding flowers to top of cake, adding more leaves as needed. Overlapping the flowers builds height and makes this cake look layered and complex. Add sugar pearls to the centers of yellow flowers. Slice and serve.