Preheat oven to 350°F. Grease and line three 6-inch cake pans with parchment paper and set aside.
In a standing mixer with a paddle attachment, combine eggs, sugar, oil, and vanilla extract until combined.
In a separate bowl, whisk together flour , baking powder, lemon zest, poppyseeds, and salt. Alternate pouring the flour mixture and buttermilk into the wet ingredients, mixing only until combined.
Divide batter evenly among prepared pans and bake for 25 minutes, or until a knife inserted in the cake tester comes out mostly clean. Allow cakes to cool completely before frosting.
To make frosting, cream butter and cream cheese in a standing mixer fitted with a paddle attachment. Slowly add in powdered sugar and milk and mix for about 30 seconds on high speed.
Frost the cake, adding about 1 cup of frosting in between each layer, using the remaining frosting for the outside of the cake. Sprinkle poppyseeds around the sides of the cake, then decorate with edible flowers!