In a large bowl, cream the softened butter, packed brown sugar, and granulated sugar until the mixture is light and creamy. This takes about 2 to 3 minutes with a mixer on medium speed.

Add the large egg and beat until fully combined. Scrape the bowl once to make sure everything is incorporated.

Mix in the creamy peanut butter and pure vanilla extract until the dough is smooth and even in color.

Gradually incorporate the all-purpose flour, baking soda, and salt. Mix just until combined, avoiding overmixing. Chill the dough for at least 1 hour to firm it up and make it easier to shape.

Preheat the oven to 325°F (163°C). Prepare mini muffin pans by greasing them lightly so the cookie cups release easily after baking.

Form cookie cups by scooping chilled dough into the prepared mini muffin pan. Press the dough down and create a shallow indent in the center of each scoop to form the cup shape.

Bake the cookie cups for 14 to 15 minutes, or until the edges are lightly golden. Rotate the pans halfway through baking if your oven has hot spots.

Allow the cookie cups to cool in the pan for a few minutes, then transfer to a wire rack to cool completely.

If the centers have puffed, gently press them down while still warm to reshape the cups.

For the fudge filling, melt the chopped semi-sweet chocolate with the teaspoon of vegetable oil or coconut oil in short bursts in the microwave or using a double boiler until smooth.

Then fold in the mascarpone cheese until the mixture becomes a glossy, smooth fudge.

Spoon the fudge filling into each cooled cookie cup and top with the optional Heath toffee bits or chopped peanuts for crunch.

Chill briefly if you prefer a firmer filling.