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1
Preheat the oven to 400F with a rack in the center position.
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2
Coat four 4-ounce ramekins or muffin cups with pan spray.
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3
If using ramekins, set them on a rimmed baking sheet.
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4
Melt the chocolate, butter, and 1 1/2 tablespoons water in a double boiler, or in a microwave oven, until you can stir the mixture smooth.
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5
Whisk in the salt, yogurt, egg substitute, and vanilla.
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6
In a small bowl, stir together the cocoa powder and flour.
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7
Using a standing mixer fitted with the whisk attachment, beat the egg whites on high speed until they are foamy.
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8
(Alternatively, use a handheld electric mixer.)
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9
With the mixer running, gradually add the sugar, a few teaspoons at a time.
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10
After all of the sugar is in, continue to mix until soft, glossy peaks form, curling over when you lift the beater.
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11
Fold the chocolate mixture into the egg whites until mostly combined.
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12
Sift half of the cocoa mixture over the top and fold it in with a spatula.
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13
Once it is mostly incorporated, sift the remaining cocoa mixture over the top and fold until it is incorporated.
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14
Spoon 1/4 cup of the batter into each prepared ramekin.
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15
Place 1 teaspoon of the peanut butter in the center of each one, and cover the peanut butter with a heaping tablespoon of batter.
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16
Bake for 15 minutes, or until a toothpick inserted into the cake (not the center) comes out clean.
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17
Set the ramekins on a wire rack and let them cool for several hours.
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18
To serve: Serve the cakes in the ramekins.
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19
If you used muffin cups, unmold them before serving.
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20
Store any leftover peanut butter cups, tightly wrapped, at room temperature for up to 24 hours.
