Preheat the oven to 350F. Combine flour, baking powder and salt in a bowl. Blend in butter until mixture resembles coarse meal. Mix in 1 cup sugar. Stir in fresh yuzu or meyer lemon peel.
Blend beaten eggs with milk, add sour cream and combine with dry ingredients until just mixed. Combine lightly with walnuts.
Pour into a buttered and floured loaf pan. Bake until a toothpick placed in the center comes out clean--roughly 1-1/4 hours.
Boil yuzu juice and 5 tablespoons sugar for 5 minutes, or until thickened. Pour this mixture evenly over the hot cake, then cool thoroughly.
I replaced the grated yuzu peel with Meyer lemon peel as I couldn't find fresh yuzu, and used pure Yuzu juice. Yuzu juice is readily available online and at local Japanese markets. If you are lucky enough to find both fresh yuzu and juice, by all means use them!