Microwave the butter in a microwave-safe mug for 20 to 40 seconds, or until it has melted. Note: I prepare the cookie dough in a small bowl or a measuring cup first and then use a rubber spatula to transfer it over to a microwave-safe mug or ramekin. You can easily just use a mug for the whole process, which requires less cleanup.

Add the peanut butter, pure maple syrup, egg yolk, and vanilla extract and mix well until everything is well-combined (wet ingredients). Note: If your peanut butter is very cold from having been sitting in the refrigerator, microwave it along with the butter to soften it up so that it stirs easily.

Stir in the gluten-free all-purpose flour and sea salt (dry ingredients) and mix until a smooth batter forms. The mixture will be relatively thin and will not resemble regular cookie dough - this is normal! If you’d like, sprinkle the top with chocolate chips or mix some chocolate chips into the cookie dough.

Microwave on high heat (the microwave’s default setting) for 45 to 60 seconds, or until it has set up. I recommend starting with 45 seconds then microwaving longer only if need be. Allow the mug to cool down for a minute or two before enjoying the peanut butter cookie - initially it will be very hot! The texture will look slightly crumbly but the cookie will melt in your mouth. Consider serving microwave peanut butter cookies with a scoop of ice cream or a glass of milk.