Prepare the streusel by combining the flour, brown sugar, cinnamon, and softened butter, in a mixing bowl. Use a large spoon, silicone spatula, or your hands to crumble the mixture until it looks like wet sand. Set aside.
Preheat the oven to 350°F. Grease a 5-inch by 9-inch (or similar) loaf pan with butter or line it with parchment paper.
Finely chop your apple and discard the core (no need to peel the apple). Set aside.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a separate mixing bowl, use a hand mixer to cream the softened stick of butter with the brown sugar and granulated sugar until it is soft and fluffy, after 1 to 2 minutes.
Add the eggs and vanilla to the creamed butter. Mix until it is creamy.
Add the dry ingredients into the wet ingredients in three additions, mixing slowly with a large spoon or silicone spatula after each addition, until there is no dry flour visible. Fold in the chopped apples.
Pour the batter into the greased loaf pan. Sprinkle the streusel on top.
Bake for 55 to 70 minutes, until a toothpick inserted into the center of the bread comes out clean. (It’s okay if crumbs stick to the toothpick, you just don’t want to see wet batter, otherwise it needs to bake longer). Let the bread cool for at least 30 minutes, then slice and enjoy!!