Preheat oven to 350℉. Grease and flour three 8'' cake pans and line the bottoms with parchment paper. Set aside.

In a medium mixing bowl, combine the flour, baking powder, baking soda, and salt. Whisk to combine and set aside. In a large mixing bowl, combine the eggs, granulated sugar, and brown sugar and whisk until smooth. Add the butter, vegetable oil, vanilla, lemon zest, sour cream, and milk and whisk until combined.

Add the dry ingredients to the wet and mix with a rubber spatula until just combined. Pour the batter evenly into the pans and smooth out the tops with a spatula. Bake for 22-25 minutes or until a toothpick inserted comes out clean. Let the cakes cool for 10 minutes, then remove them from the pans and let cool the rest of the way on a cooling rack.

In a medium saucepan, melt the butter over medium heat. In a medium bowl, whisk together the lemon zest, lemon juice, eggs, salt, and sugar. Pour a little bit of the melted butter gradually into the egg mixture, very slowly, whisking thoroughly after each addition. (This is to "temper" or bring the mixture to temperature gradually, so that the eggs don't scramble!) 

Once butter is fully incorporated, add mixture back into the saucepan. Cook over low heat until thickened, 10-12 minutes, stirring constantly. Be patient, it thickens up in the last few minutes. Be sure not to let the mixture boil. Remove from the heat and let cool completely. Can be refrigerated for up to two weeks in an airtight container.

Bring a pot of water to a boil. Add lemon slices and boil for 1 minute. Quickly remove them and place them into an ice bath to stop the cooking process.

Combine sugar and water in a medium saucepan. Bring to a boil, reduce heat, and simmer until sugar is dissolved, about 3 minutes. Add the lemon slices in 1 layer and simmer until transparent, about 45 minutes. Using a slotted spoon, letting excess syrup drip off, transfer the oranges slices to a wire rack to cool completely.

Place raspberries in a food processor or blender and purée until smooth. Strain mixture through a fine mesh strainer into a bowl. You may need to help push it through with a spatula or spoon. This should give you about 4 1/2 tablespoons of raspberry juice. Set aside.

Add the softened butter to the bowl of a stand mixer and whip on medium high for 2-3 minutes until light and fluffy. Reduce speed to low and gradually add the powdered sugar until combined. Add the vanilla, a pinch of salt, and half of the raspberry juice and increase speed to medium-high and whip for 2-3 minutes. Add the remaining raspberry juice and whip for another 1-2 minutes or until super fluffy.

To assemble the cake, place one layer on a plate or the cooling rack (just so you don't mess up the plate you are serving on, or the cake stand!), and spread with a layer of lemon curd, leaving 3/4'' or so on the edges. Top with a blob of raspberry buttercream in the center and smooth it out into a circle (this will help keep the cake layers from moving all over the place because the lemon curd is super slippery!) Top with another cake layer and spread top with lemon curd and frosting again. (You will have lots of leftover lemon curd!) Top with the third layer and spread with a layer of raspberry buttercream, leaving a little bit left for the sides of the cake. Use the leftover frosting to "half" frost the sides, to create a "semi-naked" effect. Use an offset spatula or bench scraper to smooth out the sides of the cake. Garnish cake with the candied lemon slices, raspberries, and flowers if desired. Serve!