1/3cupheavy cream or milkdon't add it all at once. Do it little by little so that you can get your desired consistency
Preheat to 325. Grease and flour 3 – 6 inch round cake pans. Set aside.
In a medium size bowl whisk the cake mix and the poppy seeds. Set aside.
In a large bowl combine the eggs, vegetable oil, sour cream, buttermilk, vanilla, and lemon zest. Whisk until everything is thoroughly combined. *
Add the cake mix to the wet ingredients. Using a spatula, mix the cake mix until combined. Don’t over mix! Alternatively, you can use a stand mixer (like I did). See recipe notes for details.
Divide the cake batter evenly amongst the cake pans.
Bake for 31-34 minutes or until a toothpick inserted in the center comes out clean; and when the top of the cake is touched it and gently springs back. Mine baked for 33 minutes. All ovens are different, so keep an eye on yours.
Cool on a wire rack in the pans for 5-10 minutes, then remove the cakes from the pans and allow to cool completely.
Once cool, frost the cakes. Or, you can double wrap the cakes individually in plastic wrap and refrigerate or freeze.
Cream butter until smooth. Add powdered sugar, vanilla extract, and heavy cream (or milk) one tablespoon at a time. Once reached your desired consistency, whip on medium high speed for 1-2 minutes.
*You can also use a stand mixer for this recipe if you'd like. Rather than whisking the wet ingredients, use the paddle attachment and mix until evenly combined. Then add the cake mix and poppy seeds to the wet ingredients and mix on low speed until just combined! Don't over mix. You can bake this in 2 - 8 inch round cake pans. Bake for 25-27 minutes or until a toothpick inserted in the center comes out clean.