Place butter and chocolate 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.
Add egg yolks and flour to chocolate mixture; whisk until smooth.
In a bowl, combine egg whites and cream of tartar. Beat with a mixer on high speed until whites are foamy. Continue to beat, gradually adding sugar, until whites hold stiff, glossy peaks.
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.
Bake in a 350°F 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).
Slide a sharp knife between torte and pan. Remove pan rim.
Cut torte into wedges and transfer to plates.
Spoon whipped cream onto each serving and garnish with berries.
Note: If making ahead, cool cake, cover airtight, and chill up to 1 day. For best texture, let cake stand at room temperature about 1 hour before serving.