1 vanilla bean, split lengthwise, seeds scraped and reserved
Instructions
In a small saucepan, combine blackberries, 1 tablespoon water, and lemon juice. Cook over medium-low heat, stirring occasionally, until blackberries release their juices. Strain mixture through a fine-mesh sieve into a small bowl, discarding solids. Set aside to let cool.
In the top of a double boiler set over simmering water, whisk together sugar and egg whites. Cook, stirring occasionally, until mixture reaches 140˚ on a candy thermometer.
Immediately pour egg white mixture into the bowl of a stand mixer. Beat at high speed until mixture is cooled, thick, white, and glossy, about 10 minutes. Gradually add butter, 1 tablespoon at a time, beating until butter is combined before the next addition. Beat in blackberry mixture and reserved vanilla bean seeds. Use immediately or store, covered, at room temperature for up to 1 day.
Recipe by Victoria at https://victoriamag.com/white-chocolate-cupcakes-vanilla-blackberry-buttercream-recipe/