Make the Swiss meringue buttercream: Combine egg whites and sugar in the heatproof bowl of a stand mixer. Heat over a pan of simmering water, whisking frequently, until sugar is dissolved and mixture reaches 160 degrees. Transfer bowl to the stand mixer and whisk on high speed until stiff peaks form and meringue has cooled, 5 to 7 minutes. Reduce speed to medium and add butter, one tablespoon at a time. Whisk to emulsify.
Switch to the paddle attachment. Add black-currant jam and beat on low speed until combined. Season with salt to taste.
Keep at room temperature if using the same day, or store in refrigerator up to week. Return to room temperature before using.