Chop the butter into cubes to help melt down more easily. Add butter and nut butter to a pan over low heat and allow the butter to fully melt.
Once melted, remove the pan from the heat to avoid burning the butter and use a whisk to break down the nut butter until fully incorporated into the butter.
Return the pan to the stove over medium heat and stir constantly with the whisk. The mixture should start to bubble and foam at the surface--continue whisking when you achieve this.
The mixture will start to develop a golden brown colour and smell rich and buttery. You should also see some solids start to form at the bottom of the pan. Once the mixture starts to deepen in colour, lower the heat and keep an eye on it because it will go from toasted to burnt very quickly. Once you're happy with the intensity of the mixture, immediately transfer it to your mixing bowl and allow it to cool.