Preheat oven to 375°F. Spray two 9x5-inch baking pans with no-stick cooking spray; set aside.
Cook potato in 1 cup water in a small saucepan for 15 minutes, until tender. Mash potato and measure with cooking water to make 1 cup. Stir in currants and butter. Cool to lukewarm.
Sprinkle yeast in 1/2 cup warm water in large bowl. Stir until yeast dissolves; stir in potato, eggs, sugar, and butter. Stir in 2 cups flour, salt, and mace until smooth. Beat in remaining 2 1/2 cups flour to make a stiff dough. Turn dough onto floured surface and knead. Let rise 1 hour.
Punch dough down, then divide the dough in half. Shape both halves into balls and set aside to rise 45 minutes until double in bulk.
After second rise, shape both halves into loaves about 9 inches in length. Place two loaves, side by side, in a prepared baking pan. Place the 2 remaining loaves, side by side, in other prepared baking pan.
Place both baking pans in oven, and bake at 375°F for 30 minutes. Let cool in pan for a few minutes, then run a butter knife around the edge to loosen bread from pan edges. Turn loaves onto a platter to cool.
Thoroughly mix the corn syrup, water and vanilla. Brush the top of the bread with the glaze while the bread is cooling.