Heat oven to 180ºC . Lightly grease an 20 cm square baking pan. CAKE: Mix flour, sugar, cocoa, baking soda and salt in a medium size bowl. Add yogurt, oil and eggs and whisk until well blended. Scrape into prepared pan. Bake 40 minutes or until pick inserted in center comes out clean. Cool in pan on wire rack 15 minutes, then turn out on rack and cool completely. MOUSSE: While cake bakes, put one third cup of the cream and the peanut butter chips in a small saucepan. Stir over low heat, until chips melt and mixture is smooth. Remove from heat, stir in vanilla and let cool 20 to 25 minutes. Beat remaining two third cup cream in a small bowl with electric mixer just until soft mounds form when beaters are lifted. (Don't overbeat or mousse won't be smooth.) Gently stir about one third of the whipped cream into peanut butter mixture until blended. Then add peanut butter mixture to remaining whipped cream and gently stir until blended and not streaks remain. Cut cake in 2 layers, using a long serrated knfie. Place 1 layer on serving plate and spread with mousse. Top with remaining cake layer. Refrigerate while preparing glaze. GLAZE: Stir cream, butter and sugar in a small saucepan over medium-high heat until sugar dissolves. Bring to a boil and add chips. Remove from heat; let stand 5 minutes, then stir until smooth. Pour glaze over top of cake and gently spread to edges, letting glaze drip down sides. Refrigerate at least 2 hours until set, or up to 1 day. Before serving, garnish with halved peanut butter cups. Calories 445; 28 g fat Exchanges 2 starch, -20ºC ruit, 5.5 fat.