Mix all-purpose flour, buckwheat flour, and baking powder together in a bowl. Cut in shortening with 2 knives or a pastry blender until the mixture resembles coarse crumbs.
Beat egg yolk lightly in a separate bowl. Mix in milk and molasses. Pour into the flour mixture; mix batter until no dry spots remain.
Beat egg white in a separate bowl until stiff. Fold into the batter.
Heat a lightly oiled griddle over medium heat. Pour batter into 4-inch cakes on the griddle. Cook until bubbles form and the edges are dry, 3 to 4 minutes. Flip and cook until browned on the other side, 2 to 3 minutes. Repeat with remaining batter.