Preheat oven to 350 degrees F.

Butter a standard muffin tin and dust with cocoa powder; tapping out excess.

In a glass measuring cup, whisk together cocoa powder and hot water until smooth.

In a medium bowl, whisk together flour, baking soda, baking powder, and salt.

In a small medium saucepan over medium-low heat, melt butter with sugar, stirring until combined.

Remove from heat and pour into the bowl of an electric mixer fitted with the paddle attachment.

Beat on medium-low speed until mixture is cool, about 5 minutes.

Add eggs, one at a time, beating until incorporated.

Add vanilla and cocoa mixture; beat until combined.

Reduce speed to low.

Add flour mixture in two batches, alternating with the sour cream, beating until just combined.

Divide batter evenly among muffin cups, filling each 2/3 full. Bake 16-18 minutes, or until a toothpick inserted into the center comes out clean.

Transfer to a wire rack to cool 15 minutes; invert cupcakes onto rack and cool completely.

In an electric mixer fitted with the paddle attachment, beat butter until light and creamy, about 4 minutes.

With the mixer on low speed, slowly add powdered sugar.

Gradually increase the speed and mix for another 4 minutes.

Add peanut butter and salt.

Beat until fully combined and frosting reaches the desired consistency.

To make the ganache, place chopped chocolate in a medium heatproof bowl.

Heat cream and corn syrup just to a simmer in a small saucepan over medium-high heat.

Pour hot mixture over chocolate.

Let stand one minute, then stir until chocolate is completely melted and mixture is smooth.

To assemble, cut each cupcake in half horizontally, such that you have a top and bottom layer.

Sandwich frosting between the top and bottom cupcake layer.

Spoon the ganache over the cupcakes and sprinkle with crumbled peanut butter cups.

Serve immediately.