MAKE THE DOUGH:

• Place butter, salt, sugar, and milk in a small saucepan; heat on low for a few minutes until lukewarm and the butter has almost melted. 
• Transfer to bowl of a stand mixer. (The mixture should be lukewarm: 100 - 110degrees F)
• Add vanilla and lemon extract or zest. 
• Add eggs; mix with a spatula to combine.
• Add 3 1/2 cups (420g) flour and yeast and mix to combine. Cover and rest for 10 minutes to absorb the flour.
• Knead with the dough hook on low speed or by hand for 5 minutes; adding 1 tablespoon of flour at a time from the remaining 1/2 cup (60g) flour as necessary. The dough should be tacky but not sticky.
• Scrape onto a lightly floured counter and knead by hand for 1 - 2 minutes to make a soft, elastic dough.
• Form it into a ball. Place in a clear container sprayed with vegetable oil; press down to flatten (so you can tell when it has doubled in size) and spray the top of the dough.
• Cover tightly with plastic wrap or a lid and place in a warm spot for 60 - 90 minutes until double in size or refrigerate overnight and bring to room temperature before proceeding.
• Press down to deflate; fold one side and then the opposite side over to form a rough rectangle. Turn out onto a lightly floured counter. Press down to get rid of air bubbles.
• Roll into a rough rectangle just under 1/4-inch thick. Use a 4 1/2-inch (11cm) round cookie/pastry cutter dipped in flour to cut as many rounds as possible. 
• Combine the scraps; cover with plastic.
• Place rounds on parchment covered baking sheets; brush with egg wash. (you only need it on the perimeter but it’s easier to just brush the whole thing)
• Place 2 teaspoons of filling/just under 1 tablespoon (Size 60 small cookie scoop) in the center of each round. (Don’t overfill)
• Push the side closest to you up with your thumbs and bring in the two sides toward it to form a triangle. Pinch very well to seal. Use your thumb, forefinger and middle fingers to pinch the middle closed.
• TURN THEM OVER, leaving 1 - 2 inches between them. Roll over gently to flatten a little.
• Cover loosely with plastic and set aside to rise 45 to 60 minutes until puffy or refrigerate overnight and bring to room temperature before baking. Reroll and repeat with the scraps.
• Twenty to thirty minutes before baking, preheat the oven to 325°F/160°C CONVECTION/FAN or 350°F/180°C REGULAR.
• Brush gently with egg wash.
• Sprinkle Cheese buns with streusel, Nutella or Poppy with sugar and Apple with cinnamon sugar.
• Bake in the middle of the oven for 13 - 17 minutes until golden brown. Rotate the baking sheet after about 10 minutes and place on a second baking sheet if the bottoms are getting too brown. Cover loosely with foil if the tops are getting too brown.
• Cool 5 minutes on baking sheets and remove to cooling racks to cool completely.