Step 1

Spray the inside of a microwave-safe 2-quart 8-inch square baking dish with cooking spray.

Step 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. Break up some of the larger brown sugar clumps with your fingers. Make a well in the center, add the buttermilk and oil and stir until everything is just combined. Scrape the batter into the prepared baking dish with a rubber spatula. Smooth the top.

Step 3

Put 1 cup water in a microwave-safe 2-cup or larger measuring cup. 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. 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. Pour over the batter in the baking dish. Do not stir in.

Step 4

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. (If the cake is slightly wet in the center, that's OK; it will continue cooking as it sits.) Let cool for at least 30 minutes.

Step 5

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. Serve the cake warm or at room temperature with ice cream.