To make your cake:

1. Preheat oven to 350. Grease and flour a loaf pan.
2. In a medium bowl, combine flour, baking powder, and salt. Mix well, then set aside.
3. In a large bowl, whisk together yogurt, sugar, eggs, lemon zest, lemon juice, and vanilla.
4. Slowly add dry ingredients to wet mixture and stir until well-combined.
5. Fold vegetable oil gently into batter until the batter is smooth/shiny and evenly mixed.
6. Scoop about 1 cup of the batter out and into a small bowl. Set aside.
7. Pour the rest of your batter into your prepared loaf pan.
8. Add your blueberry preserves to the small bowl of batter you scooped out earlier. Stir until batter is uniformly blue/purple.
9. Using a large spoon, scoop spoonfuls of your purple batter into your loaf pan, on top of the main batter (doesn’t have to be uniform or pretty).
10. Gently run a butter knife through the batter in your loaf pan (in an up/down wave motion, starting on one end of the pan and moving across the pan to the other end) to create swirls in your batter. Set swirled batter aside and get started on your streusel topping.
11. In a small bowl, mix your melted butter, light brown sugar, flour, and cinnamon until mixture is crumbly and has the consistency of wet sand.
12. Sprinkle mixture evenly over the top of your swirled batter in the loaf pan.
13. Bake for about 40-45 minutes, or until a knife in center of cake comes out clean.
14. Cool completely.

To serve:

1. Cut your cake into slices and serve with whipped cream or an extra dollop of blueberry preserves.