Position rack in middle of oven. Preheat the oven to 350°F/180°C. Line two baking sheet pans with parchment paper; set aside.

Stir the ground flax seeds and water together in a small bowl and let sit for 5 minutes to thicken slightly. This is your “flax egg”.

Place the peanut butter, sugar, and “flax egg” in a medium-size bowl and beat with an electric mixer on high speed or beat vigorously with a wooden spoon until blended well. Scrape down bowl once or twice.

For Plain: Use two spoons or a small ice cream scoop to drop large walnut sized cookies evenly spaced on prepared sheet pans. Make a crisscross impression on top of the dough with a fork, pressing down gently as you go. Cookies should be flat and fairly even in thickness. Bake for 10 to 15 minutes or until cookies are dry on the surface and just a tad soft when touched. The bottoms should be golden. Take care when checking as they are fragile at this time. They will firm up upon cooling. Place pans on racks to cool completely. Store the cookies in an airtight container at room temperature for up to 4 days.

For Jam-Filled: Use two spoons or a small ice cream scoop to drop large walnut sized cookies evenly spaced on prepared sheet pans. Now you need something small and round to make circular impressions in the dough. You could use your thumb or forefinger but I like to use the round handle of a wooden spoon. Any which way, make a small round imprint down in the dough, going about halfway down. The circle should be about 1/2-inch (12 mm) across. Bake for 10 to 12 minutes or until cookies are beginning to take on a little bit of color. The impressions might have filled in somewhat. Simply re-press and use two spoons to scrape/scoop about 1/2 teaspoon of jam into the depressions. Return to the oven for about 2 to 5 more minutes or until cookies are done. They should be dry on the surface and just a tad soft when touched. The bottoms should be golden. Take care when checking, as they are fragile at this time. They will firm up upon cooling. Place pans on racks to cool completely. Store the cookies in a single container in an airtight container at room temperature for up to 4 days.

For Chocolate Chip: Beat miniature chips into the prepared dough, then proceed as follows. Use two spoons or a small ice cream scoop to drop large walnut sized cookies evenly spaced on prepared sheet pans. Make a crisscross impression on top of the dough with a fork, pressing down gently as you go. Cookies should be flat and fairly even in thickness. Bake for 10 to 15 minutes or until cookies are dry on the surface and just a tad soft when touched. The bottoms should be golden. Take care when checking as they are fragile at this time. They will firm up upon cooling. Place pans on racks to cool completely. Store the cookies in an airtight container at room temperature for up to 4 days.