Preheat oven to 350°F, with an oven rack in the middle of the oven. Grease an 8 or 9 inch round cake pan with butter (or cooking spray). Line the bottom with a round of parchment paper, and lightly flour the pan by sprinkling some flour in the pan and tapping the pan to evenly coat the sides with flour. If making cupcakes, put 12 muffin liners in a standard 12-cup muffin tin. See notes* for cake pan size options.
In a small bowl, whisk together the eggs, vanilla, and sour cream.
Using either a stand mixer with the paddle attachment or a handheld electric mixer and a large bowl, mix the flour, sugar, baking powder, baking soda, and salt on low until combined. With mixer on low, add the butter one piece at a time. Beat until mixture resembles coarse sand.
Add half of the wet ingredients and mix on low. Increase speed to medium and beat until incorporated, about 30 seconds. Add the rest of the wet ingredients and mix on low until just combined. Increase speed to medium and beat for 20 seconds. Batter may still be a little lumpy. Scrape the sides of the bowl with a spatula and mix it a few more times with the spatula.
Scrape the batter into the prepared pan (or pans) and smooth top to spread in evenly. If making cupcakes, fill the liners about half full. Bake for 30-35 minutes if using an 8 inch pan, reducing the bake time if using smaller pans. For cupcakes, check after 16-20 minutes. Bake until the cake is golden brown and a toothpick comes out clean when inserted in the centre of the cake (or cupcake). Watch it carefully and do not over-bake because dry cake is an abomination.
Let cool in the pan for 30 minutes on a wire rack, then tip cake out onto rack, remove parchment paper, and let cool completely. Frost once completely cooled, or wrap tightly in plastic wrap and refrigerate or freeze until ready to frost.