Transfer the strawberries, the milk, the vanilla extract, the sweetener and the salt to a blender and blend on high until smooth. You may have to stop and mix with a tamper (or a spoon). Taste and adjust the quantity of the sweetener, then blend for a few more seconds.
If using strawberry puree (read notes) add 1-2 tablespoons on each glass. Then pour the milkshake into the glasses and garnish with a whole strawberry on the rim.
Notes
To make the strawberry puree, just blend 1 heaped cup of fresh strawberries (200 grams) with ¼ cup granulated sugar (50 grams).
To make the whipped cream, you will need to whip ½ of a cup very cold heavy cream with 2 teaspoons of powdered sugar, until stiff peaks form (these quantities are for 2 milkshakes).
Instead of frozen strawberries you can also use fresh strawberries. Chill them in the fridge or even better in the freezer for a few hours before using them.
If you plan to add whipped cream on top, don't fill the glass all the way up because the milkshake will overflow (you can see that from the photos).
If you don't have a high duty blender, it will be easier to blend the milk with half of the frozen strawberries until smooth, and then add the rest of the strawberries.
The milkshake will have a stronger strawberry flavor if you use almond milk. However the texture is creamier and smoother with regular milk (read the "Variations - Substitutions" section for more details).