Cookie Dough:

In a large mixing bowl, whisk all purpose flour, cocoa powder, baking soda, and fine sea salt.

In a separate mixing bowl or the bowl of a stand mixer, whip butter, granulated sugar, and brown sugar on high speed until light and fluffy, about 1 minute. Add egg and vanilla extract and mix until well combined.

Gradually add the dry ingredients, mixing on low speed until just combined, with no dry spots remaining.

With your hands, form the dough into a ball. Wrap in clingfilm/plastic wrap and refrigerate, at least 2 hours, or overnight.

Meanwhile, prepare the homemade caramel.

Homemade Caramel:

Line a 5 x 9-inch loaf pan with parchment, allowing a 1-inch overhang on the longer sides. Spray generously with nonstick cooking spray.

In a small saucepan, gently warm the cream, butter, and salt over low heat. Do not boil.

Meanwhile, in a large saucepan combine granulated sugar, water, and corn syrup and cook, periodically brushing down sides of pan with a damp pastry brush, until mixture turns medium caramel in color and reads about 365° F (185° C) on a candy thermometer.

Remove from heat. Pour in the warmed cream and butter mixture in a thin stream, then wait for a moment until the bubbles subside.

Return to heat and cook until caramel reaches 250° F (120° C) on a candy thermometer.

Pour into prepared pan. Allow to set at room temperature until firm enough to slice, at least 1 1/2 hours, or overnight.

Assembly:

Preheat oven to 350° F (175°C). Line a half sheet rimmed baking tray with parchment or a silicone baking mat.

Lift caramel slab from pan and peel away the parchment. Arrange on a chopping board. With a sharp chef’s knife, slice into 1-inch squares. (You’ll have a few pieces left over for snacking.)

With a your hands or a cookie scoop, divide the cookie dough into 12 portions. With your hands, roll each portion into a smooth ball, about 1 3/4 inches in size.

With your thumb, make a deep impression in each ball.

Shape each square of caramel into a ball, about 3/4 inches in size. Press a ball of homemade caramel into each impression.

Shape dough around the caramel until caramel is completely hidden.**

Arrange balls on prepared tray, allowing 2-3 inches of space in between for the cookies to spread as they bake.

Flatten slightly with the palm of your hand, then press 4-5 caramel M&Ms into the top of each ball. (If you enjoy a gooey, messy cookie, you can also press a few pieces of leftover caramel into the tops of the cookies after baking.)

Bake in preheated oven for 10-12 minutes, or until just barely set. Do not overbake. For perfectly round cookies, immediately reshape each cookie using a round cutter.

Sit tray on a wire rack and allow cookies to cool completely.***

Store caramel M&M cookies at room temperature in an airtight container, separated by sheets of wax paper or parchment, for up to 5 days.