whisk egg yolks,1/4cup sugar, butter, 1/4 cup water(possibly whipping cream) and coffee liqueur in large bowl.

set bowl over saucepan of simmering water

whisk until thick and frothy about 3-5 minutes

add chocolate and stir until melted and smooth. then remove from heat

use an electric mixer and beat egg whites in another large bowl to soft peaks. gradually add the remaining 1/4 cup sugar beat until stiff and glossy peaks form.

remove chocolate from over the water. fold 1/3 of the meringue into the warm chocolate to lighten then fold in remaining meringue.

cover tight with plastic wrap and refrigerate for at least 3 hours

I like putting the mouse on top of Brownies or chocolate cake