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1
Preheat the oven to 350 degrees F.
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2
For the mashed sweet potatoes: Cover the sweet potatoes with cold water in a medium saucepan and simmer over medium heat until fork-tender, 12 to 15 minutes.
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3
Drain off the excess water and transfer the potatoes to the bowl of a stand mixer fitted with the paddle attachment.
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4
Add the milk, cardamom and 4 tablespoons of the brown sugar and mix on medium until smooth, 3 to 4 minutes.
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5
Taste and add more brown sugar if desired.
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6
Set aside to cool.
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7
Spray a 9-by-13-inch baking dish with cooking spray.
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8
For the cake layer: In a clean bowl of a stand mixer fitted with the paddle attachment, cream together the egg and brown sugar until fluffy and thick, about 4 minutes.
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9
Add the yogurt, oil, vanilla extract and orange zest and mix until well combined.
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10
Add the flour, baking powder, baking soda and salt.
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11
Mix until the batter just comes together; it will be thick.
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12
Spread the batter into the bottom of the greased baking dish.
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13
Cover the batter evenly with the sweet potato mixture.
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14
For the crumble: Combine the flour, brown sugar and melted butter in a small bowl.
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15
Mix with your hands until the mixture forms a rough crumbly texture.
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16
Sprinkle over the sweet potato and bake until the bottom is set and the crumb topping is golden brown, about 30 minutes.
