Blade grinders generate excess heat that, in addition to chopping the beans unevenly, prompts them to partially cook.The pull, or extraction, must be executed properly to prevent burning.
Water Temperature
Use an instant-read thermometer to make sure the temperature of the filtered water stabilizes between 197.6 degrees Fahrenheit and 204.8 degrees Fahrenheit. Ideal temperature is imperative to prevent burning and under- or over-extraction. Although water temperatures vary slightly between machines, if yours is heating outside the aforementioned temperature gradient you should have it serviced.
Grinding and Dosing
Seven grams of coffee comprises one shot. Dosing refers to transferring the coffee from the grinder to the portafilter. The ideal coarseness of the grind varies with the amount of water pressure produced by the espresso machine, so consult the manufacturer’s recommendations. After dosing, settle the coffee grounds with a firm tap. Even distribution is paramount to even extraction. Run the unsharpened side of a knife blade over the portafilter to clear off the heaped grounds, similar to measuring flour in baking and pastry. Espresso purists however, feel nothing other than running a curved pinky proximally over the portafilter is acceptable.
Tamping
Tamp the coffee down in the portafilter with 5 pounds of pressure. You can press down on a scale with the tamper to provide a feel of what 5 pounds of pressure with a tamper feels like. Tamping, like grinding, has an undervalued importance when avoiding burnt espresso -- Again, it contributes to an even, unburned extraction. The second and third tamps need an ideal pressure of 30 pounds and 50 pounds, respectively. Polish the grounds, referred to as a pellet after tamping, by rotating the portafilter 720 degrees as you perform the final tamp.
Pulling
Ready a timer and position a demitasse cup under the group head. Pull the lever from the top position to the bottom position. The pull should last between 23 and 29 seconds. Precise pulling time is crucial to not burning the shot. Too long, and a taste similar to carbonized bitters results -- a sign of burnt espresso. Observe the extraction. The tails should have a dark brown appearance and get lighter towards the end of the pull.
Check out this related video from Homesteady on Youtube.