Melt butter and chocolate in double boiler. Set aside.
Whisk egg whites from medium to high speed to form with peaks.
Beat yolks, vanilla extract and brown sugar at medium speed in another mixing bowl till thick and fluffy. Slowly pour yolk mixture into egg white, beating at low speed. Add melted chocolate and mix well.
Sift flour ingredients in 3 batches and fold well with a rubber spatula. Add yogurt and rum, mixing well.
Pour chocolate batter into a 11 x 7.5 inch cake pan lined with parchment paper at the bottom and sides.
Mix chocolate topping ingredients with finger tips into crumbles. Sprinkle evenly over batter.
Bake in preheated oven at 170 deg C for about 40 minutes. Leave cake to cool in pan, unmould and cut into pieces. Decorate as desired.
Recipe by Anncoo Journal at https://www.anncoojournal.com/yogurt-brownie_24/