Preheat oven to 350 degrees.
Grease three 8 inch round cake pans.
Line bottoms with wax paper.
Melt chocolate in top of double boiler over simmering water; stir until smooth.
Cool.
Cream butter until light and fluffy.
Gradually add 1 1/4 c sugar, beating until smooth.
Beat in egg yolks 1 at a time.
Mix in vanilla.
Stir in chocolate, then peanuts.
Sift flour over batter and fold in.
Beat egg whites with cream of tartar to soft peaks.
Gradually add remaining 1/4 c sugar and beat until stiff but not dry.
Fold 1/2 of whites into batter, then fold in remaining whites.
Divide batter among prepared pans.
Bake until tester inserted in center comes out clean, 25 - 30 minutes.
Cool 10 minutes; turn out onto racks and peel off paper.
Cool to room temperature.

Frosting:
Beat yolks in bowl until pale yellow and ribbons form when beaters are lifted, 5 minutes.
Stir sugar and water in heavy saucepan over low heat until sugar dissolves.
Increase heat and boil without stirring until candy thermometer registers 234 to 240F.
Beat hot syrup into egg yolks in slow steady stream.
Continue beating until thick and cool, 10 minutes.
Melt chocolate in top of double boiler over simmering water; stir until smooth.
Remove from over water and let stand until cool but still liquid.
Gradually beat butter into yolk mixture 1 T at a time.
Add chocolate and stir to combine.
Transfer 2 c buttercream to small bowl, stir in Snickers bars.
Place one cake layer on platter top side down.
Spread half of Snickers buttercream over.
Top with second cake layer.
Spread remaining buttercream over.
Top with third cake layer.
Spoon 1 c plain buttercream into pastry bag fitted with medium star tip.
Frost top and sides of cake with remaining buttercream.
Serve at room temperature.