Spray an 8x8x2-inch cake pan with nonstick spray; set aside.
2.
In a medium mixing bowl, using an electric mixer on high speed, beat eggs until custard-like consistency, about 4 minutes, until doubled in volume.
Slowly add sugar, 1 tablespoon at a time, beating well after each addition.
Stir in liqueur.
3.
In a small bowl, combine remaining ingredients except chocolate chips.
Add to egg mixture, stirring and folding just until combined.
Pour into cake pan and bake for 20 minutes, or until a toothpick comes out clean from the center.
Remove to wire rack and cool in the pan.
4.
In a small heat-proof measuring cup, combine the chocolate chips and 1 tablespoon of water and melt over low heat stirring constantly until smooth, about 30 seconds.
Spread over cake with spatula.
While chocolate is still warm, sprinkle cake with walnuts 5.