• Preheat oven to 350 degrees F.
• Line a sheet cake pan with parchment paper and lightly grease, set aside.
• In a medium bowl, combine flour, baking soda, baking powder, and sea salt. Set aside
• Whisk the melted butter, sugar, eggs, sour cream, milk and vanilla in a large bowl until the mixture is smooth.
• Add the dry ingredient mixture to the wet ingredients and whisk until combined and there are no lumps.
• Evenly spread the batter into the prepared sheet cake pan.
• Then, bake the vanilla sheet cake in the preheated oven for 28-33 minutes, or until a toothpick inserted in the center of the cake comes out clean or with a few crumbs.
• Place cake pan on a wire cooling rack to cool to room temperature (takes about 30 minutes).
• Once cool, you can either lift the cake out of the pan and transfer it to a rectangular serving platter, or leave it in the pan and frost.

Make the Vanilla Buttercream

• In the bowl of a standing mixer fitted with the paddle attachment, or in a large bowl with a handheld mixer, beat the butter, vanilla and sea salt for 1 minute.
• Add 2 cups of the powered sugar, and beat until combined.
• Then, add the milk and 3 more cups powdered sugar (you should be up to 5 cups by this time), and beat until smooth.
• Add the additional 2.5 cups powdered sugar and beat until smooth and fluffy.

Assemble the Vanilla Sheet Cake

• Let the cake cool completely before spreading the vanilla frosting evenly over the cake.
• Then chill until the frosting has hardened (about 30 to 60 minutes).

Serve

• Once the frosting has set, cut the cake into pieces and serve slightly chilled or at room temperature.
• You can put the lid on the cake pan (or cover it in foil or plastic wrap) and chill it overnight if desired.