1-1/2 cups (262 g) Callebaut semi-sweet chocolate callets*
3/4 cup (171 g) egg whites, at room temperature
3/4 cup (75 g) granulated sugar
1/4 tsp cream of tartar
3/8 tsp Diamond Crystal kosher salt
3/4 lb or 3 sticks ­­(340 g) unsalted butter, softened to room temperature
1-1/2 tsp pure vanilla extract
1-1/2 tsp instant espresso powder, dissolved in 1-1/2 tsp. water
1-1/2 T (22 g) dark rum, optional

*Note: Callets are shaped like chocolate chips but are actually made of fine chocolate. If you don’t have access to callets, buy a good quality chocolate bar and chop it up fine.

Place the chopped chocolate or callets in a heat-proof bowl set over a pan of simmering water, making sure that the bottom of the chocolate bowl does not touch the water. Stir until melted and set aside until cooled to room temperature.

In the bowl of an electric mixer fitted with a whisk attachment, mix the egg whites, sugar, cream of tartar and salt. Place this bowl over the pan of simmering water (again, don’t let the bottom touch the water) and heat the egg whites, stirring frequently, until they are warm to the touch, about 5 minutes. Return the bowl to the electric mixer and whisk on high speed for 5 minutes, or until the meringue is cool and holds a stiff peak.

Add the softened butter to the egg whites, a little at a time, while beating on medium speed. Once incorporated, scrape down the bowl, add the melted chocolate, vanilla, espresso and rum, (if using) and mix for one minute or until the chocolate is completely blended in. If the butter seems very soft, refrigerate for fifteen minutes and beat it again. Or if it looks like it is separating as you beat, be patient, it will come together again.