In large bowl, beat egg whites with cream of tartar until soft peaks form. Add 1/2 cup (125 mL) of the sugar, 2 tablespoons (25 mL) at a time, beating until stiff peaks form.
In another large bowl, beat egg yolks and remaining sugar for about 4 minutes or until pale and thickened. Beat in vanilla. Combine flour and baking powder; stir into yolk mixture. Fold in one-quarter of the egg whites until combined; fold in remaining egg whites. Fold in pecans. Scrape batter into 10-inch (4 L) tube pan. Bake in centre of 325°F (160°C) oven for about 50 minutes or until firm to the touch.
Invert onto legs of pan. Let cool completely. Remove cake from pan. Cut cake horizontally into 3 layers.
Icing: In bowl, whip cream and sugar. Using 1 cup (250 mL) of the whipped cream, spread over top of bottom layer. Place second layer on top and repeat. Top with remaining layer; spread remaining cream over top and sides. Refrigerate for at least 30 minutes or up to 24 hours.
Butterscotch Sauce: In saucepan, whisk together brown sugar, cream and butter; bring to boil. Let cool completely. Drizzle some of the sauce over cake. Serve remaining sauce alongside.