Place a 11" cast iron skillet (or other oven-safe skillet) into the oven to heat for 10 minutes, then remove.
In a large bowl, beat together the eggs, brown sugar, milk, and vanilla. Then add the flour and cocoa powder and beat again until smooth.
Melt the butter in the hot skillet, and swirl to coat the skillet evenly. Pour the batter into the skillet and then bake for 15-20 minutes, or until the pancake is puffed in the middle and firm around the edges.