1. Place chocolate and butter in a microwave-safe bowl. Heat in a microwave oven at 50 percent power, stirring twice, until chocolate is soft, 3 to 4 minutes total. Remove from oven; stir until mixture is smoothly blended.
2. Add egg yolks and flour to chocolate mixture; whisk until smooth.
3. In a bowl, combine egg whites and cream of tartar. Beat with a mixer on high speed until whites are foamy. Beat, gradually adding sugar, until whites hold stiff, glossy peaks.
4. Add about a fourth of the beaten whites to chocolate mixture; stir to mix. Add remaining whites and gently fold into chocolate mixture until evenly incorporated. Scrape mixture into an 8-inch cake pan with removable rim. Spread batter smooth.
5. Bake in a 350° oven until torte puffs slightly and center barely jiggles when pan is gently shaken, 18 to 20 minutes. Cool at least 1 hour in pan on a rack (torte sinks slightly).
6. Slide a sharp knife between torte and pan. Remove pan rim.
7. Cut torte into wedges and transfer to plates. Spoon whipped cream onto each serving and garnish with berries.