PRALINE OR TOFFE ICE CREAM CAKE: Preheat over to 150ºC . Lightly oil 8.5 x 4.5 x 7 cm loaf pan. Sift first 5 ingredients into medium bowl. Using electric mixer, beat butter and sour cream in large bowl until well blended. Gradually add sugar and beat until well blended. Gradually add sugar and beat until mixture is light and fluffy. Add eggs 1 at a time, beating well after each addition. Mix in half cup sweet potato puree and vanilla. Stir in dry ingredients Transfer to prepared pan. Bake until tester inserted into center co9mes out clean, about 1 hour 15 minutes. Cool cake in pan on rack 30 minutes. Cut around sides of pan with knife. Turn cake out onto rack and cool. SAUCE: Combine milk and cinnamon sticks in heavy medium sauce pan Scrape in seeds from vanilla bean ; add bean. Bring to boil. Remove from heat. Cover and let steep 15 minutes. Wisk yolks and sugar in bowl to blend. Slowly beat milk mixture into egg mixture. Strain custard into same saucepan. Cook over medium-low heat until custard tickens and leaves path on back of spoon when finger is drawn across, stirring constantly, about 15 minutes; do not boil. Pour custard into bowl. Whisk in sour cream. Cover and chill until cold, stirring occasionally. Cut cake into slices and transfer to plates. Top with ice cream and sauce.