Measure 1 pound of frozen strawberries into a bowl.
Place 1 pound of frozen strawberries into a food processor and process until finely diced.
Scoop the processed strawberries into a small saucepot and place on the stove on medium heat.
Stir occasionally until the strawberries thaw and are easily stirred. Add the 3/4 cup of sugar and stir well.
In a small bowl, mix together the cornstarch and water. Add this mixture to the strawberries and sugar mixture.
Stir occasionally until the mixture comes to a bubble. At this point it will be noticeably thicker. Once it's the consistency of jelly, remove from heat. It will thicken more as it cools.
Pour into a separate bowl to cool.
Make the cupcakes:
Preheat the oven to 350 degrees F. and line a muffin pan with 16 liners or you can bake as a cake in two 6 inch cake pans.
In a medium bowl, melt 1 stick of butter in the microwave.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a separate bowl, whisk together the milk, sour cream, vanilla, and 1/3 cup of the cooled strawberry filling.
Add the 1/2 cup of sugar to the bowl with the melted butter and whisk. It will still be grainy.
Add 1 egg and whisk again.
Add 1/2 of the milk mixture and half of the flour mixture and mix just until combined.
Add the rest of the milk mixture and flour mixture and mix just until the big lumps are gone.
Using a large cookie scoop or 1/4 cup measuring cup, fill the cupcake liners. I can get 15-16 cupcakes out of this batter.
Place into the oven for 16-18 minutes or until they spring back when lightly touched. If baking as a cake, bake for 24-30 minutes or until they spring back when lightly touched in the middle.
Place on a cooling rack when done.
Make the buttercream:
With a handheld or stand mixer, cream the room temperature butter and salt until lighter in color. 2-3 minutes.
Add the powdered sugar and 2 tablespoons of heavy cream. Mix until well incorporated. About 2 minutes. Scrape down the bottom and sides of the bowl.
Add 1/3 cup of cooled strawberry filling. Mix well, until fluffy. About 2 minutes. Scrape down the bottom and sides of the bowl. If you feel like the buttercream is too stiff, you can add up to 2 more tablespoons of heavy cream and mix again.
Assemble the cupcakes:
Using a knife or large piping tip, scoop out a piece of the cupcakes. Make sure to not go all the way to the bottom of the cupcake. I press a large piping tip about halfway down into the center of the cupcake and remove.
Save this piece to cover the filling.
Place at least 1 teaspoon of filling into the empty cavity of the cupcakes.
Push the reserved pieces of cupcakes that were scooped out, back onto the filling. It's ok if it's slightly messy as it will be covered with buttercream.
Scoop or pipe as much of the strawberry buttercream onto the tops of the cupcakes as you'd like!
This recipe will work baked as a cake in two six inch cake pans!