Step 1: Make the Streusel Topping

In a medium bowl, mix together the flour, sugar, cinnamon, and salt.

Stir in the melted butter until the mixture resembles crumbs.

Place the streusel in the refrigerator while you prepare the cake batter.

Step 2: Prepare the Cake

Preheat your oven to 350°F (175°C). Line a 9 x 5-inch loaf pan with parchment paper, leaving a 2-inch overhang for easy removal. You can also choose to spray the loaf pan with non-stick baking spray.

In a medium bowl, whisk together the cake flour, baking powder, and salt. Set aside.

In a mixing bowl, beat the butter and vegetable oil on medium speed until light and fluffy (about 2 minutes).

Gradually add the sugar in three additions, mixing until fluffy.

Add the egg and vanilla, blending well.

Alternate adding the dry ingredients and buttermilk, starting and ending with the flour mixture. Scrape down the sides of the bowl as needed.

Gently fold in the blueberries, then pour the batter into the prepared loaf pan.

Sprinkle the streusel topping evenly over the batter.

Step 3: Bake the Bread

Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.

If the top browns too quickly, cover it loosely with aluminum foil for the last 10 minutes.

Allow the bread to cool in the pan for about 30 minutes, then lift it out using the parchment paper and let it cool completely on a wire rack. Dust with powdered sugar if you desire.