Mix the flour, dark brown sugar and salt in a mixing bowl. Add the butter to the dry ingredients and mix on low speed until butter is well combined. In a separate bowl add the buttermilk, vanilla and egg to the warmed milk and whisk well. Stir in the yeast and pour into the dry mixture. Mix well on low speed until dough comes together and doesn't stick to the sides of the bowl, about 5 min. Remove dough from mixing bowl onto a lightly floured clean surface and knead 10 min or until no longer sticky. (If dough is very sticky, add flour as necessary.) Return to mixing bowl, cover with plastic wrap and leave in a warm place for 1 hour to rise.
Meanwhile, combine the granulated sugar with the cinnamon. Set aside 1/3 cup (75 mL) of the cinnamon sugar mixture in another bowl. Grease a non-stick Bundt pan. Divide the dough into 45 equal pieces and roll into rounds (approx. 1” in diameter). Pour 1/4 cup (60 mL) of the melted butter into a bowl. Dip each ball into the melted butter, then roll in cinnamon sugar and place at the bottom of the Bundt pan. Arrange half of the pieces in the bottom of the pan.
Reheat remaining butter in a saucepan, add the remaining 1/3 cup (75 mL) cinnamon sugar, brown sugar, apples and pecans. Mix well until sugars dissolve. Spoon half of the apple-pecan mixture over the dough pieces in the Bundt pan. Finish rolling the rest of the pieces in butter and cinnamon sugar and place in Bundt pan. Spread remaining apple-pecan mixture over top. Drizzle any remaining butter and cinnamon sugar mixture over the loaf. Cover with plastic wrap and then foil and refrigerate overnight. (Note: If baking the same day, cover the loaf as directed and allow to rise for a minimum of 1 hour at room temperature.)
Before baking, remove from fridge and place in a warm place to rise for 1 hour. Preheat oven to 350°F (180°C). Remove plastic wrap and foil from Bundt pan. Grease foil layer and place back over top of pan. Bake for 20 min, then remove foil and bake for another 25 to 30 min or until top is a dark brown. Cool for 2 to 3 min, then invert onto cooling rack. Keep inverted until loaf is almost completely cool. Invert onto a plate to serve.