Preheat the oven to 350°F (180°C) and position a rack in the center of the oven. Grease a 9x5-inch loaf pan with butter or non stick spray. Or line with parchment paper.
In a large bowl, whisk together the flour, baking powder, salt, and granulated sugar. Set aside. 2 cups (240 g) all purpose flour 1 Tablespoon baking powder ½ teaspoon salt 1 cup (200 g) granulated sugar
In a second large bowl, whisk together the eggs, milk, oil, and vanilla extract until well combined. 2 large eggs ⅔ cup (160 ml) milk ½ cup (120 ml) vegetable oil 2 teaspoons vanilla extract
Add the wet ingredients to the dry ingredients and mix until just combined—do not overmix. Stop mixing once the last of the flour has been incorporated.
In a small bowl, stir together granulated sugar and cinnamon. ½ cup (100 g) granulated sugar 1 tablespoon ground cinnamon
Pour half of the batter into the prepared loaf pan and spread it evenly. Sprinkle half of the cinnamon sugar mixture evenly over the batter. With the tip of a knife, swirl the cinnamon sugar mixture throughout the batter.
To the remaining half of the cinnamon sugar mixture, stir in the melted butter and flour and stir until it becomes crumbly. 2 tablespoons unsalted butter 2 tablespoons all-purpose flour
Pour the remaining batter over the cinnamon sugar layer in the pan and gently spread it out. Sprinkle the streusel mixture on top.
Bake for 50–55 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pan for 10–15 minutes. Then remove from the loaf pan and cool on a wire rack.
In a small bowl, whisk together the powdered sugar, milk, and vanilla until smooth. Drizzle over the slightly cooled bread before slicing and serving. 1 cup (113 g) powdered sugar 1-2 tablespoons milk ½ teaspoon vanilla extract