Set your oven to 350°F (175°C). Line a 10x15-inch jelly roll pan with parchment paper and lightly grease it.
In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt until fully combined.
In a large bowl, beat eggs and granulated sugar for 3 minutes until thick and pale. Stir in vanilla and milk.
Gently fold dry ingredients into the egg mixture using a spatula, mixing just until combined. Avoid overmixing.
Pour batter into the prepared pan, spread evenly, and bake for 10–12 minutes, or until the center springs back when lightly pressed.
Invert the warm cake onto a powdered sugar–dusted towel. Peel off parchment and roll up the cake (with the towel). Let it cool completely.
Beat peanut butter, powdered sugar, butter, and vanilla in a bowl until smooth and creamy.
Carefully unroll the cooled cake and spread the peanut butter filling evenly over the surface.
Re-roll the cake without the towel and place it seam-side down on a serving plate.
Heat heavy cream until simmering. Pour over chocolate chips, let sit 2 minutes, then whisk until smooth.
Spread ganache over the cake roll and sprinkle chopped Reese’s cups while the ganache is still soft. Refrigerate for at least 1 hour before slicing for neat, firm pieces.