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Preheat the oven to 350°F with a rack in the center position. Lightly grease an 8’’ x 8’’ pan.
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To make the filling: In a small bowl, toss together the flour and black sesame seeds. Set aside. (You’ll use the halva later.)
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To make the streusel: In a medium bowl, use your fingers or a spoon to thoroughly combine all the ingredients, mixing until crumbly. Set aside.
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To make the cake: In the bowl of a stand mixer, use the flat beater to beat the butter, sugars, vanilla, baking powder, and salt at medium-high speed until pale and fluffy, about 3 minutes. Stop and scrape down the sides of the bowl as needed.
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Add the eggs one at a time, beating at medium-high speed for 1 minute and scraping down the sides of the bowl after each addition.
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On low speed, stir in about 2/3 cup of the flour followed by roughly 1/2 cup of the sour cream. Add the remainder of the flour, then the rest of the sour cream, mixing just until combined; don’t overmix.
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To assemble and bake the cake: Transfer about half the batter into the prepared pan, spreading to the corners.
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Sprinkle the filling evenly across the batter and top with the halva. Resist the urge to poke the halva down into the batter.
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Spread the remaining batter on top of the halva, and top with the streusel.
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Bake the cake for 40 minutes, then tent it with foil to prevent the top from over-browning. Bake for an additional 5 to 10 minutes, until golden brown on top and a knife inserted in the center comes out clean. You may have melted halva or small crumbs stuck to the knife, but you shouldn’t see any wet batter.
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Remove the cake from the oven and set it on a rack to cool for at least 1 hour before cutting and serving.
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Storage information: Store the cake, securely covered, at room temperature for several days; freeze for longer storage.
