Preheat the oven to 400 degrees F.

Butter 4 ramekins and then dust the inside with powdered sugar.

For the dark chocolate part: combine the chopped dark chocolate with the coffee, 1/4 cup of butter and melt it. Either au-bain-marie or in the microwave.

For the white chocolate part: combine the chopped white chocolate with 1/4 cup of butter and melt it. Either au-bain-marie or in the microwave.

Once the chocolates are melted, allow them to cool down a little bit, until room temperature.

Beat the egg whites with the cream of tartar until the egg whites start to become stiff; then add 2 tablespoons of powdered sugar and beat until you have soft peaks.

Add 2 egg yolks to each chocolate mixture and combine.

Divide beaten egg whites over the chocolate mixtures and combine.

Pour both chocolate mixtures at the same time to fill the small ramekins.

Place ramekins on a baking sheet and bake for approx. 10 minutes. The outside of the souffle with be baked, but the inside will remain soft and liquid.

Dust the top of the ramekins with powdered sugar and serve immediately. You can leave the souffle in the ramekins or you can take them out of the ramekins.