Dough: In a small bowl combine 1-2 tablespoons water, baker's yeast and sugar. Mix and leave to bubble up (10-15 minutes).
In a medium sized bowl add flour, salt, baking powder and mix everything. Make an indentation and add the yeast mixture, oil and water. Knead ingredients several minutes until you get a soft ball of dough. Leave dough to rise for an hour.
Filling: (Make while dough is rising) In a medium sized pot add milk and bring to a boil on medium. Bring temperature down to low, add sugar, vanilla extract and ground poppy seeds. Mix well until integrated.
While continuously stirring, add semolina. Continue stirring and cooking another 1-2 minutes. Turn the stove off and let the filling rest. When the filling cools down divide it into two equal parts.
Assembly: Heat oven to 355°F (180°C) and dust working area with flour if needed. (Sometimes the dough is greasy enough where this isn't necessary. Your call.) The dough should be doubled now, so cut it into two equal parts.
Take one dough half and roll it out into an (approximately) 8 by 16 inches rectangle. Then take one half of the filling and spread evenly over the dough rectangle almost all the way to the sides. Roll up the long side and place in a pan (sized approximately 12 by 17 inches) lined with parchment paper. Repeat with the other halves.
Mix water and butter then brush poppy seed rolls with it.
Bake for 10-15 minutes, then turn the pan around and cover the roll with parchment paper. Bake an additional 25 minutes. (Total baking time is 40 minutes.)
Take the pan out of the oven, and let it cool down. You can cover it with a clean, dry kitchen towel. Dust with confectioner's sugar before serving.
The roll keeps up to 3 days. Keep wrapped in saran wrap in bread box. Eat leftover potica cold or reheated.