Preheat oven to 350° F. Line large baking sheets with baking parchment paper.
Combine butter and sugar in a bowl. Whip with a handheld mixer or stand mixer on medium-high speed until fluffy.
Combine in egg and vanilla. Beat until fully blended.
Add dry ingredients in two additions, mixing on low speed until just combined.
Scrape down the sides of the bowl with a spatula and cover the dough with plastic wrap and refrigerate for at least 30 minutes (this helps the dough hold the indentation shape when baking).
Place chopped walnuts into small bowl. * the walnut steps are optional. You can skip the nuts, if you prefer.
Scoop the dough into 1” balls. A round tsp scoop works well. Roll between your palms into an even ball. Try to handle the dough as little as possible.
Roll in walnuts before placing on baking sheet. If walnuts are having trouble sticking, they may be too big and need to be chopped a bit more finely (be sure to pop the dough in the fridge if you find you need to chop them more finely).
Repeat with remaining dough, placing balls 2” apart.
With your thumb (or knuckle) press a ½” indentation in each ball.
Scoop some jam into each indentation until they are slightly heaped, about ¾ tsp jam.
Bake for 14-15 minutes until golden brown. If baking two sheets at once, rotate sheets halfway through the bake time and bake for an extra 1-2 minutes.