Line a 13" x 9" baking pan with a piece of parchment paper, so that it covers the bottom and the sides. The parchment paper sheet should overhang on two sides sides of the pan.
In a medium saucepan over medium-low heat, combine the butter, sugar, and salt. Stir until all of the butter has melted. Don't rush this process by bumping up the heat!
Bring the mixture to a gentle boil. Stir occasionally until it reaches about 300° F on a candy thermometer or until it reaches a dark amber color. Immediately remove from heat when it reaches the desired color and temperature.
Stir in the baking soda. The mixture will foam some. Pour the mixture into the prepared baking pan. Spread the mixture out as evenly and as thinly as possible (or to your desired thickness). Allow it to set for 20 minutes at room temperature.
When the toffee has cooled, take it out of the pan using the parchment paper overhang and place it onto a clean cutting board. Dab any remaining butter from the toffee's surface using a paper towel.
To absorb the remaining butter, sprinkle the cocoa powder over the toffee's surface using your fingers to rub it in fully.
Before the toffee has finished setting, place the chocolate in a microwave-safe bowl and microwave for 30 seconds. Remove the bowl from the microwave to stir. Continue this process of microwaving by 30-second intervals and stirring until chocolate has completely melted.
Spread the chocolate evenly over the toffee's surface. Sprinkle with crushed pretzels on top. Cool to room temperature for about 20 minutes.
Cover it gently with aluminum foil, and place toffee in the fridge to chill until set.
Remove from the refrigerator and allow it to reach room temperature. Break or cut the toffee into 1.5-inch pieces. Store in an airtight container for up to two weeks.
Serving Size 1.5-inch square of toffee