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1
Mix the brown sugar, oats, flour, and salt in a medium bowl until evenly combined.
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2
With your fingertips, blend in the butter pieces until small clumps about the size of a dime form and the butter is well incorporated, about 2 minutes.
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3
Refrigerate the streusel while you make the cake.
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4
Heat the oven to 350 degrees F and arrange a rack in the lower third.
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5
Coat a 10-inch springform pan with butter.
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6
Line the bottom of the pan with parchment paper, coat the parchment with butter, and set the pan aside.
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7
Rinse the blueberries and dry off any excess water (but leave the berries slightly damp).
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8
Transfer to a medium bowl, add 2 tablespoons of the flour, and toss to coat; set aside.
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9
Whisk the remaining 2 cups flour, baking powder, cinnamon, salt, and nutmeg together in a medium bowl until evenly combined; set aside.
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10
Stir the milk and vanilla together in a small bowl or measuring cup until evenly combined; set aside.
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11
Place the measured butter in the bowl of a stand mixer fitted with a paddle attachment.
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12
Mix on medium speed until smooth and light in color, about 2 minutes.
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13
Add the sugars slowly, about 2 tablespoons at a time, until both have been added, about 5 minutes total.
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14
Stop the mixer and scrape down the sides of the bowl and the paddle with a rubber spatula.
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15
Return the mixer to medium speed and add the eggs one at a time, letting the first completely incorporate before adding the second.
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16
Scrape down the sides and paddle again.
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17
Reduce the speed to low, add a quarter of the flour mixture, and mix until just combined.
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18
Pour in a third of the milk mixture and mix until just incorporated.
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19
Repeat, alternating dry and wet ingredients and ending with the dry ingredients, until completely incorporated.
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20
(Do not overmix or the cake will be tough.)
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21
Remove the bowl from the mixer, add the reserved blueberries (leaving any excess flour behind), and, using a rubber spatula, fold in the berries until evenly distributed.
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22
Turn the batter into the prepared pan.
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23
Sprinkle the chilled streusel evenly over the batter and lightly press it in with your hands.
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24
Bake until a tester inserted into the center of the cake comes out clean and the top is golden brown, about 55 to 60 minutes.
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25
Remove the pan from the oven and let it cool on a wire rack for at least 20 minutes.
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26
If the sides of the cake have not pulled away from the pan by then, carefully run a knife around the outer edge of the cake.
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27
Unlock and remove the outer ring.
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28
Let the cake cool for 30 minutes more.
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29
To remove the bottom of the pan, place a large piece of foil over the top of the cake and gently fold it around the sides.
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30
Place a cooling rack or plate on top and invert the cake.
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31
Remove the springform bottom and peel off the parchment paper, then place a cooling rack or serving dish on top of the cake.
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32
Invert the cake again, remove the foil, and serve.
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33
The cake can be stored in an airtight container at room temperature for up to 2 days.
