Preheat oven to 350°F and prepare two greased, parchment-lined 8-inch cake pans. *9-inch baking pan works, too. Adjust your baking time to less.
Whisk together the flours, baking powder, baking soda, and salt.
Beat butter, oil, and sugar until light and fluffy, about 3–5 minutes.
Add eggs one at a time, mixing well after each addition.
Mix in the lemon zest, lemon juice, vanilla, and sour cream until just combined.
Alternate adding the dry ingredients and buttermilk, beginning and ending with the dry ingredients. Mix only until incorporated.
Divide the batter evenly between the prepared pans and smooth the tops.
Bake for 28–32 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Cool in the pans for 10–15 minutes, then transfer to wire racks to cool completely.
For the optional syrup, heat the lemon juice and sugar until dissolved, cool, and brush lightly over the cooled cake layers.
To make the frosting, beat the butter until creamy. Add the cream cheese and beat until smooth. Gradually mix in the powdered sugar, then add the lemon zest, lemon juice, vanilla, and salt. Add heavy cream if needed for a spreadable consistency.
Place one cake layer on a serving plate and spread about 1½ cups of frosting over the top. Add the second layer, apply a thin crumb coat, chill for 20 minutes, then frost the entire cake.
Garnish with fresh lemon slices, extra zest, or berries, if desired. Chill for 20–30 minutes before slicing for the cleanest cuts.