In a small bowl, combine yeast with lukewarm water; whisk until yeast is dissolved. Let stand about 3 minutes until foamy. Add butter, salt, sour cream & potato; mix well. Add bacon & chives; mix until just combined. Stir in flour, one cup at a time. When dough is completely blended, turn onto a lightly floured surface (using some of the remaining 1/4 cup flour). Knead dough about 10 minutes, until smooth & elastic. Place dough in a lightly greased bowl, cover with plastic wrap & let rise in a warm place until doubled in size, about 1 hour. Remove risen dough from bowl, turn onto lightly floured surface; roll dough to about a 12″ X 15″ rectangle. Place a large piece of parchment on a sheet pan. Roll dough onto a rolling pin then unroll onto parchment paper. Place 1/2 of the cheese down the center of dough, top with chicken, broccoli & remaining cheese. Fold short ends in about 1″. Using parchment, fold one side over filling, overlapping with opposite side. Cover with plastic; allow to rise for 15 minutes while preheating oven to 375 F. Brush with egg wash. Bake for 25-30 minutes until golden.