In a large bowl, whisk together 2 1/2 cups flour, 1/4 cup sugar, baking powder, salt and cinnamon until combined. Using a pastry blender or your fingers, cut in the cold butter until the mixture is coarse and crumbly, with a few bits of butter sprinkled throughout.
In a large measuring cup or medium bowl, whisk together 1 cup heavy cream, egg and vanilla extract. Add to flour mixture; use a spatula to bring the ingredients together (do not overmix). Once the mixture forms a loose dough, use your hands to fold and knead the dough several times in the bowl to completely form a dough and pick up any dry bits from the bowl.
Transfer the dough to a well-floured surface. Pat the dough into a 6x14-inch rectangle. Using a pizza cutter, bench scraper or sharp knife, cut the dough into 12 equal squares. Brush each square with heavy cream, then sprinkle 6 pieces generously with streusel.
Flip the non-streusel-topped squares on top of the streusel-topped squares, cream side down. Brush the tops with more heavy cream, then top with more streusel. Stack all the pieces on top of each other. Transfer the stacks to the prepared loaf pan, cut sides facing up (like a sideways lasagna).
Brush the top of the dough with more heavy cream, then sprinkle any remaining streusel from the bowl or your work surface on top. Bake for 50 to 60 minutes or until the loaf is a deep golden brown and a toothpick inserted in the center comes out clean (you may have to place a sheet of aluminum foil over the top of the loaf while baking if it starts to get too brown).
Cool bread in pan 15 minutes, then use parchment paper to remove from pan to a cooling rack.