Preheat the oven to 350. In a bowl with an electric mixer cream the butter and gentle sweet until light and fluffy. Add the sour cream and vanilla and beat until combined. Add the eggs and mix well. Stir in the flours, baking powder, and salt until thoroughly combined. Divide batter between 12 muffin cups lined with paper liners. Bake at for 30-35 min. Let cool completely.
Stir together the peanut butter and sweetener. Divide between the cupcakes. Chill in the fridge for at least 30 minutes.
Melt the chocolate in a glass bowl in the microwave stirring every thirty seconds. Alternatively, you can melt it in a double boiler on the stove top. Warm the cream in the microwave until it is about the same temperature as the chocolate. Use an electric mixer to incorporate the cream into the melted chocolate. Add the sweetener and mix until smooth and shiny. It should be about the consistency of hot fudge.
Spoon a big blob of chocolate on top of each peanut butter topped cupcake. Gently spread it to cover the peanut butter.
Notes
You may have leftover ganache. You can use it like hot fudge, dip strawberries in it, or eat it with a spoon. If you want to avoid having leftover ganache you can make ⅔ of the chocolate recipe. I, personally, have no problem using up extra melted chocolate.Notes on Sweeteners: