Preheat the oven to 350°F (180°C). Line 6 muffin cups with paper liners.

For the cupcakes: In a large bowl, whisk together the brown sugar and cocoa powder. Add the hot coffee, and whisk until smooth. Pour in the buttermilk and oil, and whisk until combined. Add the egg yolk, tahini, and vanilla extract, and whisk until smooth. Add the flour, baking soda, baking powder, and salt and whisk just until a smooth cake batter is formed. Divide the batter evenly among the paper liners (about ¼ cup or 62 grams).

Bake until the cupcakes spring back to a light touch, about 20 minutes. Remove and set aside for 5 minutes before transferring the cupcakes to a wire rack to cool completely.

For the frosting: Add the butter to the bowl of a stand mixer and, with the flat beater attached, beat on medium speed until the butter is very pale and creamy, 2 to 3 minutes. Add the tahini, and mix briefly until combined. Slowly add the confectioners’ sugar and salt until all has been added. Increase the speed to high, and beat for about 5 minutes or until the frosting is light and fluffy. Add the vanilla and mix just until evenly distributed.

Use a small spoon to scoop out a little of the cake from the top of each cupcake to form a well. Fill each with a tablespoon of jam (20 grams), and then top with the tahini frosting, a few blackberries, and a sprinkle of sesame seeds. Best served on the day made but can be kept refrigerated for 2 to 3 days.