Preheat the oven to 350°F. Line 2 cupcake pans with cupcake liners.
Combine granulated sugar, whole milk, sour cream, vegetable oil, and eggs in a large bowl and use a hand or stand mixer to beat together until completely combined.
In a separate, small bowl, whisk together all-purpose flour, salt, and baking powder. Slowly add to the wet ingredients while beating on low speed, just until combined. Do not over mix.
Fill each cupcake liner about ¾ full with the cupcake batter.
Bake for 18 to 22 minutes, rotating halfway through, or until an inserted toothpick comes out clean.
Let the cupcakes cool completely before frosting.
In a large bowl or the bowl of a stand mixer, beat the butter on medium speed until completely smooth, about 1 to 2 minutes. Add salt and mix until combined.
Add one cup of powdered sugar and beat until combined. Add another cup of powdered sugar and beat again, until nice and smooth.
Add vanilla extract and beat until incorporated. Add 1 tablespoon of heavy cream and beat for 1 minute.
Add the remaining 2 cups of powdered sugar, adding one cup at a time and beating until smooth and creamy.
Add cream, one to two teaspoons at a time until your desired consistency is reached. Continue beating until frosting is smooth and fluffy.
Prepare a piping bag (or ziploc bag) with a star tip and fill it with the frosting.
Use a knife or another frosting tip to remove a small amount out of the center of the cupcake. You want to leave enough space for a teaspoon or so of strawberry jam.
Add 1 to 2 teaspoons of strawberry jam to the center of each cupcake. You can either spoon the jam or pipe it in for less mess.
Frost each cupcake and top with fresh strawberries if desired. Enjoy!