• Warm buttermilk and butter in microwave in 20 second intervals until butter starts to melt; let stand for 5 minutes, stirring occasionally, to finish melting (don't overheat buttermilk or it will curdle). Add egg and beat with fork.
• Whisk together 4 cups of the flour, sugar, yeast and salt in bowl of standing mixer fitted with the paddle attachment. Add buttermilk mixture and beat until dough comes together.
• Switch to the dough hook attachment and knead for 8-10 minutes, until dough is smooth and elastic, adding remaining flour as needed until dough pulls away from the sides of the bowl, but sticks to the bottom.
• Add a splash of oil to the bowl and turn dough to coat both the dough and the sides of the bowl. Cover with a clean towel and place somewhere warm to rise for 1 1/2 hours, or until doubled in volume.

To make filling:

• Combine brown sugar and cinnamon in a small bowl.

To assemble:

• On a clean, lightly floured surface, roll dough into a rectangle 15" wide and 1/4" thick. Spread evenly with softened butter, then sprinkle with brown sugar mixture and top with apples. Roll up the dough, starting at the long edge closest to you - use a bench scraper or metal spatula to help lift it if it sticks. Slice into 16 equal pieces and arrange in parchment-lined baking dishes (two 9" square, or other sizes are fine - just don't over-crowd them). Let rise 45 minutes, until nearly doubled again.
• Bake in 375ºF oven for 20 minutes, or until golden.

To make glaze:

• Whisk together all glaze ingredients and drizzle over cooled cinnamon buns.