1½ cups (300 grams) granulated sugar⅓ cup (85 grams) reserved Roasted Strawberries juice (recipe follows)⅔ cup (160 grams) egg whites (about 5 large egg whites), room temperature½ teaspoon (1.5 grams) kosher salt¼ teaspoon cream of tartar2 cups (454 grams) unsalted butter, cubed and room temperature2 teaspoons (12 grams) vanilla bean paste1¼ cups (284 grams) chopped Roasted Strawberries (recipe follows)
In a medium saucepan, combine sugar and Roasted Strawberries juice. Bring to a boil over high heat, being careful not to splash sides of pan, stirring to help sugar dissolve. (Do not stir once mixture starts to boil.) Cook until an instant-read thermometer registers 240°F (116°C).
Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment, beat egg whites, salt, and cream of tartar at medium speed until soft peaks form.
With mixer on medium speed, pour hot sugar syrup between side of bowl and whisk attachment in a slow, steady stream. Increase mixer speed to high; beat until stiff peaks form and bowl and meringue are relatively cool to the touch, 8 to 10 minutes.
With mixer on medium speed, add butter, a few cubes at a time, beating until combined. (Mixture will look very loose to start and may even look broken at one point, but buttercream will come together; see Notes.) Beat in vanilla bean paste. Beat until light and fluffy, 1 to 2 minutes, stopping to scrape sides of bowl. Fold in Roasted Strawberries. Use immediately, or refrigerate in an airtight container. Let refrigerated buttercream come to room temperature and rewhip using the whisk attachment until silky smooth before using. (See Notes.)
Notes: If the meringue is too warm when the butter is added and the mixture is too loose, place the whole mixer bowl in the refrigerator for a few minutes to help the mixture firm back up. Place the bowl back on the mixer; continue to beat as directed, repeating process as necessary.
If the butter was too cold when added and is forming small lumps in the buttercream, gently warm the mixer bowl over a saucepan of simmering water just until the edges start to melt. Alternatively, warm the exterior of the bowl using a kitchen torch just until the edges of the buttercream mixture start to melt. (Only use these methods with heatproof metal mixer bowls.) For a gentler approach, warm the bowl with a hot towel to help soften the butter. Continue to beat as directed, repeating process as necessary.
If buttercream breaks or looks curdled, keep beating it; it should eventually come together.
If next–day buttercream is still thick, lumpy, and not whipping correctly, it may still be too cold. Gently warm using one of the methods mentioned above, rewhipping with the whisk attachment until silky, smooth, and spreadable.
