Adapted from Cook's Illustrated
2 teaspoons instant coffee
1 1/2 cup water
2/3 cup Dutch process cocoa
1/3 cup brown sugar
1 cup sugar
6 tablespoons butter
2 ounces semisweet chocolate, chopped
3/4 cup flour
2 teaspoons baking powder
1 tablespoon vanilla
1 tablespoon Kahlua
1/3 cup milk
1/4 teaspoon salt
1 egg yolk
Preheat oven to 325 º. Spray 8 x 8 inch baking dish or equivalent sized dish with non-stick cooking spray. Set aside.
Stir instant coffee into water and set aside.
Whisk together 1/3 cup cocoa, brown sugar and 1/3 cup of the sugar. Set aside to put on top of batter before baking.
Melt butter and semisweet chocolate in microwave, then add remaining 1/3 cup of cocoa. Whisk till smooth and set aside to cool slightly.
Whisk together remaining 2/3 cup sugar, vanilla, milk, Kahlua, and salt in a medium bowl. Mix in egg yolk. Sift in flour and baking powder and whisk to combine. Add chocolate mixture and mix till combined. Add flour mixture and mix until all is incorporated.
Pour batter into prepared dish and spread to all corners. Sprinkle cocoa/sugar mixture evenly over batter. Pour coffee mixture over cocoa mixture. Bake until cake is puffed and bubbling, and edges are puling away from the sides, about 45 minutes. Do not overbake. Cool about 25 minutes before serving.