Combine cocoa powder, sugar, and water in a medium saucepan and heat over medium-low heat. Whisk constantly and heat until the sugar and cocoa powder are fully dissolved. This will take about 2-3 minutes.
Once dissolved, add the peanut butter and continue to cook over medium-low heat, whisking constantly, until the peanut butter is fully dissolved into the syrup. This will take about 5 minutes. Remove the syrup from the heat, add in vanilla and a dash of salt, and whisk to combine.
Let cool fully and then store in an airtight container in the refrigerator for up to 2 weeks. Shake before using.
For Peanut Butter Mocha
Place the milk into a microwave-safe glass measuring cup. Microwave for 45 seconds and then using a whisk, whisk the milk to make it frothy.
Pour ½ cup of strong coffee into a large coffee mug, add 1 to 2 tablespoons of the prepared peanut butter syrup, whisking to combine. Top with the frothed milk. Serve immediately.
Notes
For Non-Dairy Version: Use any unflavored non-dairy milk of your choice. Sweetened or unsweetened both work. Adjust syrup to taste. Use as much or as little syrup as you like to taste.