Let’s walk through the process carefully. Remember, temperature and timing are important!

1. Whip Egg Yolks: Place the egg yolks into the bowl of your stand mixer fitted with the whisk attachment. Whisk on medium-high speed for a few minutes until they become thick, pale in color, and increased in volume.

2. Prepare Sugar Syrup: Meanwhile, combine the water and granulated sugar in a small saucepan. Place over medium-high heat. Bring the mixture to a boil without stirring. Attach a candy thermometer to the side of the pan (ensure the tip is submerged but not touching the bottom). Cook until the temperature reaches exactly 238°F / 115°C (soft-ball stage). This usually takes about 5 minutes once boiling, but rely on the thermometer.

3. Combine Syrup and Yolks (Pâte à Bombe): Once the syrup reaches 238°F/115°C, immediately remove it from the heat. Turn the mixer (with the whipping yolks) down to medium speed. Very carefully and slowly pour the hot sugar syrup in a thin, steady stream down the side of the mixing bowl into the egg yolks. Avoid pouring it directly onto the whisk, which can cause splatter.

4. Whip Until Cool: Once all the sugar syrup has been added, turn the mixer speed up to high. Continue whipping the mixture for 5 to 7 minutes. The mixture will become very thick, pale, and increase in volume. Crucially, continue whipping until the outside of the mixing bowl feels just lukewarm or near room temperature to the touch. Adding butter while the mixture is too warm will cause the butter to melt and the buttercream to fail.

5. Add Butter Gradually: Ensure the butter is softened but still cool (not greasy or melty). With the mixer running on medium-low speed (you might switch to the paddle attachment here, though whisk can work), begin adding the butter, just 1 to 2 tablespoons at a time. Mix for about 15 to 20 seconds after each addition, allowing it to incorporate before adding the next piece. Scrape down the sides and bottom of the mixing bowl often.

6. Emulsify (Don’t Panic!): As you add the butter, the mixture will likely look separated, curdled, or soupy. This is normal! Keep adding the butter and continue mixing. As more butter is added, the emulsion will form, and the buttercream will start to thicken and come together into a smooth consistency.

7. Add Flavorings: Once all the butter is incorporated and the buttercream is smooth, add the melted (and slightly cooled) chocolate, salt, and coffee extract.

8. Final Mix: Mix again on medium speed for a few minutes until the flavorings are fully incorporated and the frosting is uniform in color and fluffy.

Your luxurious Chocolate Coffee French Buttercream is ready!