Preheat the oven to 350°F (175°C), grease a 9x13 inch pan and line with parchment, then cream 1 cup butter and 1 ¾ cups granulated sugar on medium-high for 4 to 5 minutes until pale and fluffy.

Add 4 eggs one at a time, beating 30 seconds after each, then mix in 1 tbsp vanilla extract.

Whisk together 2 ½ cups flour, 2 tsp baking powder, ½ tsp baking soda, and ½ tsp salt in a separate bowl, then add to the batter in 3 additions alternating with 1 cup sour cream and ¼ cup milk, mixing just until combined.

Spread the batter into the prepared pan and bake at 350°F (175°C) for 32 to 38 minutes until a toothpick comes out clean. Cool completely in the pan, at least 1 hour.

Beat 1 ½ cups butter for 3 to 4 minutes, add 5 cups sifted powdered sugar in 2 additions, then add 3 tbsp heavy cream, 2 tsp vanilla extract, and ¼ tsp salt and beat on medium-high for 3 more minutes until stiff and fluffy.

Spread the buttercream over the cooled cake, swirl with the back of a large spoon, and finish with sprinkles or sugar pearls.