Preheat oven to 350°F. Grease an 18×13 sheet pan, 9×13 pan, or two 8-inch round cake pans.
In a mixer or large bowl, beat the butter and sugar together for 3–4 minutes, until the mixture becomes light, fluffy, and pale in color.
Add the eggs one at a time, mixing well after each addition.
Mix in the vanilla, milk, and sour cream.
Add the flour, baking powder, and salt. Using a spatula, gently mix until just combined.
Spread the batter evenly into the prepared pan using an offset spatula.
Bake until the center springs back lightly and a toothpick inserted into the center comes out with a few moist crumbs.
Let the cake cool completely before frosting.
Blend thawed strawberries into a smooth purée.
In a mixer or large bowl, beat butter and salt until light and fluffy.
Gradually add powdered sugar and mix until smooth.
Add vanilla. With the mixer running on medium-low, add the cold strawberry purée 1 tablespoon at a time.
Once all purée is incorporated, increase speed to medium-high and beat 2–4 minutes until smooth, fluffy, and spreadable.
Frost the cooled cake, slice, and enjoy.
