Preheat the oven to 350°F (180°C). Grease a half sheet pan with nonstick spray and set aside.
In a large bowl, whisk together the granulated sugar, flour, baking soda, and salt. Set aside.
2 ½ cups (575g) granulated sugar, 2 ½ cups (380g) all-purpose flour, 1 ¼ tsp baking soda, ¼ tsp salt
In a separate bowl, whisk together the eggs, sour cream, and vanilla. Set aside.
3 large eggs, 1/2 cup (120g) sour cream, 1 Tbsp vanilla extract
In a saucepan over medium heat, combine the water and butter. Bring to a boil, stirring occasionally.
1 ¼ cups (295ml) water, 1 ⅓ cups (295g) butter, salted
Pour the hot mixture over the dry ingredients and mix until combined.
Whisk in the egg mixture until the batter is smooth.
Pour batter into the prepared pan and bake for 20-22 minutes, or until set.
While the cake is baking, add the milk, butter, and vanilla to a saucepan. Bring to a gentle boil, stirring frequently.
¼ cup (59ml) milk, 10 Tbsp butter, 3 tsp vanilla extract
Remove from heat and whisk in the powdered sugar until smooth. Whisk in more milk, a small amount at a time, to thin the frosting to your liking, if desired.
3 1/2 cups (375g) powdered sugar, White food coloring, if desired
Pour the warm frosting evenly over the hot cake and spread to cover evenly.
Let the cake cool completely before slicing and serving.