01 -
Combine strawberries, granulated sugar, lemon juice, lemon zest, and salt in a medium saucepan. Cook over medium heat for 8-10 minutes, stirring often, until the berries soften and release their juice.
02 -
Mash with a potato masher or blend briefly with an immersion blender. Simmer over medium-low heat for 12-18 minutes, stirring more often near the end, until thick and reduced to about 1/2 cup.
03 -
Transfer the strawberry reduction to a shallow bowl. Cool completely to room temperature before adding it to the buttercream.
04 -
Beat the softened butter in a stand mixer with the paddle attachment for 2-3 minutes, until pale, creamy, and smooth.
05 -
Add powdered sugar 1 cup at a time on low speed. Mix in vanilla and salt, then beat on medium for 2 minutes, scraping the bowl once.
06 -
Beat in the cooled strawberry reduction 1 spoonful at a time until smooth. Add the freeze-dried strawberry powder if using, then beat for 1 minute more.
07 -
If the buttercream is too stiff, add cream or milk 1 teaspoon at a time. If it is too soft, chill for 10-15 minutes, then re-whip before spreading or piping.