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1
Spray the inside of a microwave-safe 2-quart 8-inch square baking dish with cooking spray.
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2
Whisk together the flour, 1/2 cup of the brown sugar, 1/4 cup of the cocoa powder, baking soda and 1/2 teaspoon salt in a large bowl.
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3
Break up some of the larger brown sugar clumps with your fingers.
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4
Make a well in the center, add the buttermilk and oil and stir until everything is just combined.
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5
Scrape the batter into the prepared baking dish with a rubber spatula.
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6
Smooth the top.
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7
Put 1 cup water in a microwave-safe 2-cup or larger measuring cup.
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8
Microwave on high (at 100 percent power) until steaming hot, 2 minutes in an 1,100-watt oven or 3 minutes in a 700-watt oven.
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9
Add the remaining 1/2 cup brown sugar and 1/4 cup cocoa powder, the vanilla and 1/4 teaspoon salt, and whisk to dissolve the sugar.
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10
Pour over the batter in the baking dish.
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11
Do not stir in.
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12
Microwave the cake on high (at 100 percent power) until the top is mostly set and dry to the touch, 8 minutes in an 1,100- or a 700-watt oven.
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13
(If the cake is slightly wet in the center, that's OK; it will continue cooking as it sits.)
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14
Let cool for at least 30 minutes.
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15
When you cut into the cake, you will notice a gooey chocolate pudding layer at the bottom; be sure to scoop some with each serving.
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16
Serve the cake warm or at room temperature with ice cream.
