Preheat the oven to 350 degrees Fahrenheit. Prepare two 8" round cake pans by greasing and flouring them or spraying them with cooking spray. Place a parchment round in the bottom of each baking pan.
Cream the butter with the sugar in a large bowl. Beat in the eggs, 1 at a time. Beat in the sour cream.
In a separate bowl, sift the flour together with the salt and baking powder.
Add half the dry ingredients to the wet mixture, and beat thoroughly. Stir in the milk and beat again. Repeat with the remaining half of the dry ingredients.
Divide the batter between the two pans. Bake the cakes for 30 minutes or until they pass the toothpick test.
Let the cakes cool in the pans for 10 minutes, then unmold them onto a wire rack to cool completely before filling and frosting.
Cream Filling and Frosting
2 1/4 cups heavy whipping cream
3 tablespoons sugar
1 tablespoon flour
1 teaspoon vanilla extract
Warm 3/4 cup of the heavy whipping cream in a small saucepan over medium heat. Take the pan off the heat before the cream reaches a boil.
In a small bowl, whisk the sugar with the egg and flour until the mixture is frothy.
Keep whisking the egg mixture while you stir in the warm cream, 1 tablespoon at a time.
Put the cream mixture back into the saucepan and warm it over medium-high heat on the stove. Whisk occasionally until the cream reaches a healthy simmer. At that point, keep whisking until the pastry cream has considerably thickened, 5 to 10 minutes.
Take the cream off the heat, whisk in the vanilla, and let it cool at room temperature for 10-15 minutes. Then transfer the cream to a bowl, press a layer of plastic wrap against the top, and let it chill for at least four hours or overnight.
When the cream has chilled, use it to fill the layer cake.
Whip the remaining 1 1/2 cups of heavy whipping cream until it reaches stiff peaks. Frost the sides and top of the cake with the whipping cream.