The night before serving, dissolve yeast in 1 cup warm water. Add warm milk, flour and salt. Mix into a smooth batter. Cover loosely with plastic wrap and rest at room temperature overnight. Just before cooking, mix in maple syrup, baking soda dissolve in hot water, and melted butter. Stir in walnuts. Heat 2 teaspoons butter in a skillet or on a griddle until it foams. Scoop 1/4 cup batter onto hot surface for each pancake. Cook over moderate heat until the pancake is covered with bubbles. Flip and cook another minute or two, until pancake feels spring. Keep warm while preparing the rest of the pancakes, greasing the pan with more butter for every batch. Serve with syrup, fruit and honey or flavoured butter. Makes 12 pancakes.