Combine yeast and 1/2 cup warm water in small bowl. Let rest 5 minutes. Heat milk, butter and sugar until warm (100° to 110°F) but butter won’t all melt. Pour into large mixer bowl with yeast mixture and 2 cups flour; beat for 2 minutes. Add eggs, buttermilk, mashed potatoes, salt, vanilla and 1 cup flour. Beat for 2 minutes. Gradually add flour until a soft dough forms. Knead on a lightly floured surface until smooth and elastic (about 8 to 10 minutes). Place in a greased bowl, turning to coat. Cover.
Let rise in a warm, draft-free area about 1 hour, until doubled in size. Punch dough down; divide in half.
Roll one portion of dough on a lightly floured surface to a 12 x 18-inch rectangle. Spread with half the butter. Combine sugars, beverage flavourings and hazelnuts; sprinkle half of the mixture over surface. Roll up tightly lengthwise, sealing edges. Cut into 12 slices. Place in greased 13 x 9-inch pan. Repeat with remaining dough. Cover. Let rise 30 to 45 minutes until nearly doubled.
Bake in preheated 350°F oven for 25 to 30 minutes. Cool for 15 minutes. Combine icing ingredients and drizzle over rolls.