Stir white sugar, light brown sugar, evaporated milk, corn syrup, and cocoa powder in a saucepan and bring the mixture to a full rolling boil over medium heat. Reduce heat to medium low and cook, stirring constantly, until the syrup reaches 220 degrees F (104 degrees C) when measured with a candy thermometer. A small amount of syrup dropped into cold water should form a soft ball that flattens when removed from the water and placed on a flat surface.
Remove pan from heat and stir in marshmallows, vanilla extract, butter, and pecans, beating the candy by hand until creamy and the candy begins to to set up slightly at the edges of the pan, about 10 minutes.
Drop the warm candy by teaspoons onto the parchment paper; allow room for pralines to spread slightly. Cool the candy thoroughly before serving.