In a large bowl add yeast and sugar. Heat milk until warm to touch and add it in the bowl. Whisk to combine. Cover the bowl with a clean towel and let it proof for 5-10 minutes until the mixture becomes foamy (that's how you know your yeast is still active).
Add butter, 1 egg, flour and salt. Mix the dough for 5 minutes. Add 1-2 tbsp flour if it's too sticky to handle. Form into a ball, place it in a bowl greased with butter and cover with plastic wrap. Place in a slightly warm environment to rise for about 60-90 minutes or until doubled in size.
Mix the granulated sugar and ground cinnamon to make the filling. Preheat oven to 180°C and line a large baking sheet with parchment paper.
After the dough has risen, punch it down to release the air.
Divide into 4 equal pieces and place on a floured work surface. Using a floured rolling pin, roll each piece of dough into a 25 cm circle. Place the bottom circle on the baking sheet lined with parchment paper. Top with the filling - 1 tbsp very soft butter, then sprinkle ⅓ of the sugar and cinnamon mixture. Layer the remaining circles and filling on top. The last one doesn't have filling on it. Using a sharp knife or pizza cutter, cut any scraps around the edges to make an even circle. Place an 8 cm round bowl in the center and make an indent. Then, cut 16 strips from the edge to the edge of the bowl - first, cut quarters, then divide each quarter in 4 strips. Using both hands, grab two strips and twist them away from each other twice, then press the two ends together to make a point.
Cover the shaped bread with plastic wrap and let it rest for 20 minutes. To make the egg wash - beat 1 egg with 1 tbsp milk. Brush the star bread with egg wash. Bake for 25-30 minutes or until golden brown. Remove from the oven and cool for 5 minutes.
Dust powdered sugar on top and enjoy warm.