Preheat oven to 180 degrees C (350 degrees F). Line a rectangular baking pan with parchment paper and grease very well.
Peel core and cut the pears into thin slices. Arrange the slices very closely on the bottom of the pan to overlap one another.
In a small saucepan heat butter, the two kinds of sugar and soy sauce. When the mixture starts simmering, lower the heat. Stir and cook 1-2 minutes, until the sugar melts completely.
Pour the mixture over the pears. Set aside while preparing the batter.
In a bowl combine flour, cocoa and baking powder and stir.
In a large mixing bowl with an electric mixer at high speed beat eggs and the two kinds of sugar until the mixture becomes thick and foamy (about 5 minutes). Add oil, milk, vanilla and soy sauce and beat shortly at a lower speed until combined. Using a rubber spatula, gradually fold in the flour and cocoa mixture. At the end add the melted chocolate, carefully mixing with the spatula until absorbed.
Pour the batter in the pan, on top of the pears. Bake for 40-50 minutes or until a toothpick inserted in the centre comes out clean or just with 1-2 crumbs.
Let the cake cool completely inside the pan. If necessary trim slightly the top in order to become flat.
Carefully invert the pan over a rectangular serving plate. Remove the pan and the paper (very carefully because the paper will be soaked).
Keep leftovers refrigerated for 2-3 days.
Alternatively you can use a 8 inch (20-22 cm) round cake pan (not a spring form pan!), or a rectangular pan with different dimensions and volume capacity 1,5 - 2 l.