Preheat the oven to 140 degrees C. Grease a 20x30cm Swiss roll pan.
In a large bowl, cream together 1 cup of butter, white sugar and brown sugar until light and fluffy. Mix in the egg yolk and 1 tablespoon of vanilla. Combine the flour, cardamom and salt; stir into the batter until it forms a soft dough. Spread evenly in the prepared pan. Brush the top with egg white and sprinkle pecans over the top.
Bake for 1 hour in the preheated oven. Prepare the icing while the biscuits bake.
Melt the remaining butter in a saucepan over low heat until it starts to brown. Remove from the heat and stir in icing sugar until smooth. Stir in enough milk to achieve a drizzling consistency. Drizzle icing onto the bars when warm. Cool, then cut into bars.