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Preheat the oven to 350°F. Prepare an 8" square pan with a parchment sling or reusable silicone liner, then lightly grease the sides of the pan. Alternatively, line a 9" cake pan with a round piece of parchment or round reusable liner and lightly grease the sides.
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To make the crumbs: Weigh your flour; or measure by gently spooning into a cup, then sweeping off any excess. In a medium bowl, whisk together the flour, masa harina, sugars, and salt. Add the butter and vanilla; toss to combine. Using a bowl scraper, a fork, or your fingers, work in the butter until crumbs form and no dry spots remain. The crumbs should be irregular, ranging in size from a pea to walnut. Set aside.
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To make the cake: In a small bowl, combine the masa harina, water, and vanilla. Stir until smooth, then set aside to allow the masa harina to hydrate while you prepare the rest of the batter. The mixture will thicken slightly.
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In a large bowl or bowl of a stand mixer, combine the flour, sugar, baking powder, salt, and baking soda. Add the butter and mix at low speed using an electric hand mixer or the flat beater on a stand mixer, until the mixture looks sandy, about 1 to 2 minutes. (This mixing method is called reverse creaming and is also known as the paste method.)
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Beat in the eggs, then mix until no dry spots of flour remain. Stop the mixer and scrape the bowl and beater(s) as needed; the mixture will be thick.
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Add the hydrated masa harina and yogurt or sour cream all at once before starting to mix. Continue to stir until the batter is smooth. Stop to scrape the bowl and then mix again at low speed for 1 minute; this final mix is key to ensuring the best texture so don't skip it.
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Use a flexible spatula to gently fold in 1/4 cup (37g) of the blueberries into the batter by hand; try to keep the blueberries whole.
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To assemble and bake the coffee cake: Transfer the batter into the prepared pan and spread it evenly all the way to the edges. (A small offset spatula is a helpful tool here.)
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Sprinkle the remaining 1/2 cup (74g) blueberries evenly over the top of the batter, followed by the crumbs. Gently press the crumbs to adhere.
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Bake the blueberry coffee cake for 54 to 58 minutes, until it's set in the middle and the crumbs are golden brown. (Check for doneness after 54 minutes and add additional time only as needed to avoid overbaking.) The coffee cake is done when a toothpick or paring knife inserted into the center comes out with just a few moist crumbs clinging to it.
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Remove the blueberry coffee cake from the oven and allow it to cool in the pan for 30 minutes.
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Remove the blueberry coffee cake from the pan. Finish by dusting with non-melting sugar or confectioners’ sugar, if desired.
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Storage information: Store the blueberry coffee cake, well wrapped, at room temperature for several days; freeze for longer storage.
