Melt the 2 tablespoons butter in a small frying pan over medium heat.
Add sesame seeds and cook, stirring, until toasted; set aside to cool.
In a bowl, stir together flour, baking powder, and salt until well blended.
With a pastry blender or 2 knives, cut the 1/2 cup butter into flour mixture until it resembles coarse crumbs.
In a separate bowl, stir together sour cream and milk.
Add to flour mixture, blending gently.
Turn out on a lightly floured board and knead gently about 5 times.
Roll out dough to a rectangle about 15 inches long, 12 inches wide, and 1/4 inch thick.
Brush with half the beaten egg, then spread with toasted sesame seeds.
Starting with long side, roll up jelly roll fashion; pinch edge to seal.
Cut roll with a floured knife into 1 inch slices and place slices, cut side up, on a lightly greased baking sheet.
Brush slices with remaining egg.
Bake in a 425° oven for about 15 minutes or until lightly browned.