Wash strawberry and remove stem and a bit of the surrounding white flesh with a paring knife. Dry it with paper towels.
Place the strawberries and sugar in a small pot, boil on LOW heat for approx 30 minutes.
It will start to break down by itself. Occasionally, stir the mixture so the sugars and juices come together.
Using a heat proof spatula, occasionally press down on the fruit very gently to help break it down. If it is still hard, just let it break down by itself until its soft enough to push down on it with the spatula.
It will reduce into a very thick, chunky compote that is not runny. Take off heat after appr. 30 min. (Watch accompanying video to see the proper consistency. You may need to keep it on the heat some more if it is still runny.)
Pour the compote into a food processor or blender and blend for a minute or two until the fruit is all broken down.
Pour through a sieve into a heat proof cup. The mixture is thick so use a spoon to push it through. If the consistency is correct, you should have about 3 tablespoons of puree. Set puree aside.
For the Swiss Buttercream
Cut up butter and let it come to room temperature as you measure out other ingredients.
Add sugar and egg whites into a heat proof mixing bowl. Stainless steel is light and safe.
Place over a pot of boiling water.
Whisk the whites & sugar mix over the boiling water.
Pay attention to whisking the mixture when it starts getting hot, you don’t want to cook the eggs.
Whisk the mixture until the temperature reaches 160F. This temperature is generally accepted to be able to kill salmonella in eggs.
Pour the mixture into the Kitchenaid mixer and whisk on low with the balloon whisk, then turn it up to medium.
Once the mixture is thick and glossy, and the bottom of the bowl is cool to the touch, stop mixer. Change the whisk attachment to the paddle attachment.
Add butter a little bit at a time while beating with the paddle attachment on medium speed.
The mixture might look curdled or separated for a while but keep mixing. It will all come together.
Add strawberry puree a tea-spoonful at a time to a max of 3 tablespoons which is equal to 15 teaspoons. Beat until incorporated.
Add strawberry extract if desired and incorporate.