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1
Make the topping: preheat oven to 350u00b0; spray the bottom and sides of an 8-inch square baking pan with nonstick cooking spray.
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2
Combine the brown sugar, water, butter, and salt in a small pan; bring to a boil, whisking occasionally; set aside to cool.
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3
Make the cake: in a bowl, combine the flour, baking powder, cinnamon, and salt.
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4
In a large bowl, combine the butter and sugar; cream with an electric mixer on med-high speed until fluffy, about 2 minutes.
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5
With the mixer on med-low speed, add 1/3 of the flour mixture to the bowl.
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6
Add 1/2 of the milk and the vanilla; add another 1/3 of the flour mixture.
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7
Add the remaining milk and then the remaining flour; scrape down the sides of the bowl and beat on medium speed for 30 seconds.
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8
Scrape the batter into the prepared pan; smooth the top with a rubber spatula.
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9
Pour the topping over the batter (the pan will be very full); carefully transfer the pan to the oven and bake until set, about 45-50 minutes.
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10
Let the cake cook in the pan for 15 minutes, invert it onto a large rimmed serving platter, and serve warm.
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11
Let leftover squares cool completely and store in a cake keeper or airtight container at room temperature for up to 1 day; in the refrigerator for 3 days.
