Rinse and drain berries. Set aside while preparing mousse.
Heat the chocolate chips, milk, vanilla extract and espresso
powder in a small saucepan over medium-low heat until chocolate
melts. Whisk until smooth. Beat in egg yolks, one at a time,
and continue to cook and whisk until mixture thickens and reaches
140 degrees F. Remove from heat.
Place egg whites in an oven-proof glass or metal bowl, and set
in a saucepan over simmering water. Add cream of tartar and
beat until foamy. Gradually add sugar, beating constantly. Beat
until whites form soft peaks and sugar is dissolved, about 5
to 7 minutes. Stir 1/4 of the whites into the chocolate mixture
and blend. Gently fold in the remaining whites.
Spoon about 1/2 cup of berries into each of 4 stemmed glasses.
Divide the mousse among the glasses. Top with remaining berries.
Cover with plastic wrap and refrigerate until firm, several
hours or overnight.