In a large bowl using a hand mixer or a stand mixer fitted with paddle attachment, beat the butter, brown sugar, and granulated sugar on medium high speed until light and creamy, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg and beat until combined, about 1 minute. Scrape down the sides and bottom of the bowl with a silicone spatula as needed. Add the peanut butter and vanilla extract and beat until combined.

Add the flour, baking soda, and salt to the wet ingredients and beat on low speed until the dry ingredients are incorporated, and then increase to medium speed and beat until well combined. The dough will be very creamy and soft. Cover and chill the cookie dough in the refrigerator for at least 1 hour, and up to 3 days.

Grease 2 mini muffin pans (or bake in batches if you have only 1 pan) with nonstick spray, or line with mini cupcake liners. I prefer these without liners; the dough is easier to “thumbprint” without liners. Set aside.

Scoop and roll cookie dough, about 1 Tablespoon/20g of dough each. Place the dough balls in the prepared pan. With your thumb, press down in the center of each dough ball, to make an indent, and press the dough partway up the sides of the muffin cup. (If you only have 1 mini muffin pan, refrigerate the 2nd batch of dough balls until ready to bake.)

Bake for 14–15 minutes or until the edges appear lightly browned. Do not under-bake these. The cookie cups will puff up in the oven; that’s ok and expected.

Remove from the oven and place the mini muffin pan on a cooling rack. Let the cookie cups cool in the pan for just 5–10 minutes. During this time, use the back of a rounded teaspoon to lightly press into the center of each cookie cup to make the indent deeper again. After 5–10 minutes, carefully remove cookie cups from the pan. Use a butter knife to help, if needed. Place cookie cups on a cooling rack to cool completely.

Melt the chopped chocolate and oil together in a double boiler or use the microwave. For the microwave, place the chocolate and oil in a large heat-proof bowl. Melt in 20-second increments, stirring after each increment until completely melted and smooth. Let slightly cool for 3 minutes. With a silicone spatula or a spoon, gently fold the mascarpone into the melted chocolate until combined.

Spoon a heaping teaspoon of the filling into each cooled cookie cup. Top with toffee bits or chopped peanuts, if using.

Cover and store at room temperature for up to 1 day, or in the refrigerator for up to 5 days.