This recipe was given to me by a caterer friend, though you can probably find variations of it on more than a dozen websites throughout the blogosphere. She notes to avoid cake mixes with "pudding in the mix".
Line the wells of one or more cupcake tins with paper liners.
Whisk together the first 4 ingredients in a large mixing bowl. In a separate bowl, stir together the sour cream, water, eggs and extract. Add the wet ingredients to the dry ingredients and beat with an electric mixer on low speed. Scrape the bowl down and mix again. Divide the batter between cupcake liners filling them 2/3 full. Bake for 15 to 17 minutes, or until the cakes spring back when pressed in the centers.
Transfer the cakes to a wire rack to cool completely. When the cakes are cool, cut a divot in the tops of the cakes using a serrated knife. Discard the cut-out, or save it for an ice cream sundae topping. Fill each cooled cupcake with strawberry jam.
In a stand mixer fitted with the whisk attachment, whip the butter until creamy. Add the confectioners’ sugar. Begin on low speed until just combined, and then increase to high and beat for 3 to 5 minutes ,or until the frosting lightens in color and texture.
Add vanilla and beat again for another minute. If you find the buttercream is too stiff, you may add milk or heavy cream 1 tablespoon at a time until the mixture is spreading consistency.
Transfer the frosting to a large piping bag fitted with a large closed star decorator piping tip. Pipe swirls of frosting onto the tops of the filled cupcakes. Immediately decorate frosted cupcakes with confetti sprinkles, if using. Store cupcakes in a container that seals air-tight. If preparing cupcakes a day ahead of time, store them in the refrigerator in an air-tight container. Allow them to come to room temperature before serving.