In a large bowl, thoroughly whisk together all-purpose flour, cornstarch, cocoa powder, salt, and baking soda, then set bowl aside.
Using a stand mixer (or hand mixer + large bowl), cream together granulated sugar and unsalted butter on high until fluffy.
Drop mixer speed to low and add eggs; mix just enough to break yolks, about 15 seconds. Add vanilla, vinegar, and blue food coloring to bowl and mix again.
Keeping mixer speed on low, about 1/3 of the dry ingredients to the bowl. Next, add about 1/2 of the buttermilk. Add another 1/3 of the dry ingredients, then pour in the remaining buttermilk. Finish by mixing in the remaining 1/3 of the dry ingredients and stop mixing as soon as the dry ingredients are absorbed.
Pipe or spoon cupcake mixture into prepared muffin tin. Be careful to only fill them 2/3 full - cupcakes will rise significantly while baking.
Bake cupcakes for 23-28 minutes or until a tester comes out clean.
Let cupcakes cool completely before serving.
For the Real Strawberry Buttercream
In a small saucepan, add pureed strawberries and bring to a boil over medium heat. Once boiling, reduce heat so that strawberries are either simmering or heavily steaming. Let puree cook, stirring frequently, for 20 minutes or until strawberry puree is reduced by half. If you find that too much of a film is forming on the bottom of the pan, reduce heat.
Remove strawberries from heat and let cool to room temperature.
Using a stand mixer (or hand mixer + medium bowl), cream butter and 2 cups of powdered sugar on high speed until smooth. Add vanilla extract and 2 tablespoons of cooled strawberry puree, then mix again. Add another cup of powdered sugar and another 2 tablespoons of cooled strawberry puree and mix. Finally, add all remaining strawberry puree and powdered sugar to bowl and mix. If adding red food coloring, blend in 1 teaspoon at a time until desired color is reached.