In large bowl, dissolve 1/2 tsp (2 mL) of the sugar in water and sprinkle with yeast and let stand until frothy, about 10 minutes. Whisk in butter and remaining sugar. Add eggs one at a time whisking well after each addition. Add sour cream and vanilla whisk until smooth.
Stir in 3 cups (750 mL) of the flour and stir until sticky dough forms. Add 1/2 cup (125 mL) of the flour and knead into dough in bowl. Turn out onto floured surface and knead until smooth dough forms, adding any remaining flour as necessary. Place in oiled bowl; cover and let rise about 1 1/2 hours or until doubled.
Punch down dough and turn out onto floured surface and divide in 12 balls. Cut marzipan log into 12 pieces. Tuck 1 piece into centre of each ball and roll into smooth ball. Place on parchment paper lined baking sheet; cover and let rise about an hour or until doubled.
Brush tops with egg yolk and sprinkle with almonds. Bake at 400°F (200°C) in oven for about 20 minutes or until golden brown and bottom of bun sounds hollow when tapped.