NOTE: Do not preheat oven. This dough needs to be chilled at least 1 hour before baking.
Prepare dough:
Combine dry ingredients in a medium bowl and stir. Set aside.
Melt butter in a large saucepan over medium-low heat. Remove from heat.
Add brown sugar and peanut butter directly into the saucepan, stirring well after each addition.
When butter mixture has cooled somewhat, mix in egg and vanilla until smooth and creamy. (If added too early, the eggs might begin to cook in the hot butter.)
Add dry ingredients to the butter mixture in the saucepan and stir until incorporated.
Stir in optional flaxseed or peanuts, if desired.
The dough will be very soft, so you must refrigerate it 1 hour or longer before baking. You can also chill overnight.
Make and bake:
When ready to bake, preheat oven to 375°F. If using a non-stick baking dish, there’s no need to spray with oil. If not, lightly spray or rub oil onto surface.
Place granulated sugar in a small, shallow bowl or plate.
Form into 1-inch balls and roll them in the sugar. Use a fork and press down on each ball of dough in 1 direction, then the other. This flattens the cookie and leaves the classic peanut butter cookie imprints.
Bake for 7 to 8 minutes.
Remove from oven and cool slightly. Use a food turner to transfer cookies from baking sheet to a cooling rack or a piece of waxed (or parchment) paper to finish cooling. Store in an airtight container.