Place the chocolate into a medium heat-proof bowl.
In a small saucepan over medium heat, stirring constantly, bring the heavy cream just to a simmer. Stir in the peanut butter until it is smooth.
Pour the cream-peanut butter mixture over the chocolate in the bowl. Let it sit for 5 to 7 minutes until the chocolate has softened. Stir the chocolate and cream together until they are thoroughly combined.
Cover the bowl and place in the freezer for one hour until the ganache is very firm.
Once firm, scoop eight generous teaspoonfuls of ganache, pat them quickly into balls and place on a plate or tray lined with parchment paper. cover the tray lightly with plastic wrap and place in the freezer.
Preheat the oven to 350° F. Lightly grease eight wells in a muffin pan.
In a medium bowl whisk together the flour, cocoa, baking powder, and salt.
In the bowl of a stand mixer, beat together the butter and sugar until light and fluffy. Add in the egg, vanilla and cooled coffee until mixed together.
Add in milk and the dry mixture alternating in three to four additions until just combined.
Pour a little batter into each prepared muffin well. Remove the ganache from the freezer and place one ball into each well. Divide the remaining batter between the eight wells.
Bake the cakes for about 15 minutes until the cakes are firm to the touch. Remove the pan from the oven and place on a cooling rack for ten minutes.
Carefully tip the cakes onto individual serving plates. Dust each with confectioner's sugar and serve immediately.
Store leftover cakes tightly wrapped for up to four days.