Whisk 1 tsp. baking powder, ¼ tsp. baking soda, remaining 188 g Alchemy Flour or all-purpose flour, and remaining ½ tsp. salt in a medium bowl. Set aside.
Using a stand mixture fit with a paddle attachment, add in grated ginger and sugar to the mixing bowl and beat on low speed until sugar is moist and fragrant and no large clumps remain.
Add butter, room temperature, increase speed to medium-high, and beat until pale and fluffy, scraping down sides of bowl 2–3 times, about 5 minutes.
Add eggs, one at a time, beating on medium speed 1 minute after each addition until incorporated. Scrape down sides of the bowl.
Mix in sour cream and vanilla extract and beat once more to make sure batter is smooth and homogenous (don’t worry if it looks curdled). Scrape down sides of the bowl.
Add in dry ingredients and mix on low until nearly incorporated.
Add in frozen or fresh blueberries and fold in with a rubber spatula.
Transfer batter into prepared pan and spread out evenly.
Sprinkle the streusel topping over the batter. You can mash the topping between your fingers if you want smaller pieces.
Bake for 55-60 minutes, until the streusel topping is golden brown, and a cake tester inserted into the center comes out clean.
Let it cool in the pan before removing by lifting the parchment overhang.
Cut into squares before serving.