Making the streusel:
In a bowl, add all the ingredients and mix well using your fingers. Keep about three tablespoons from the streusel to the aside, to sprinkle on top of the cake if you wish.
Making the cake:
Preheat oven to 350F/180C. Line a 9 x 5 x 3-inch baking pan with parchment paper.
In a bowl, sift the flour, baking powder, baking soda, and salt. Keep aside.
In a different bowl, add the sugar, butter, oil, and vanilla, beat well for at least three minutes.
Add the egg to the previous mixture and beat until combined. Add the milk and beat for one minute.
Add the dry ingredients to the wet and gently mix for few seconds.
Pour half the batter into the baking loaf pan, spread the streusel evenly, pour the remaining cake batter on the streusel. Sprinkle what left from the streusel on top of the cake.
Bake for 45 to 50 minutes or until a skewer inserted in the middle comes out clean.
Cool for 6 minutes while in the pan, then place on a cooling rack to cool completely.