Preheat oven to 350 degrees. Put milk into a saucepan and add poppy seeds. Heat this to boiling point only and remove from the heat. Let it stand for at least 20 minutes. Cream butter with light brown sugar until fluffy. Separate eggs and add the yolks to the beaten butter and sugar. Save the whites. Add white flour, baking powder, and salt and mix well. Beat egg whites until stiff but not dry. Fold these into butter mixture, then carefully fold in poppy seeds and milk until mixture is just blended. Squirt in juice from half a lemon. Pour gently into the greased pan and bake: 35 mins for the square pan or 45-50 minutes for the loaf pan or until a toothpick comes out clean.