In a medium size bowl, cream together the softened butter and sugar until well combined. Stir in the egg and vanilla extract. Then, stir in the flour and salt. Cover and chill the dough for 1 hour or overnight.
Divide the dough into 20 pieces (dough balls should fit in the palm of your hand). Form a ball, then gently press onto a clean surface so the bottom has a flat edge.
Use the underside of a teaspoon or your thumb to create the center well. Dab your finger in water and line the edge or rim of the well center.
Place on a baking sheet lined with parchment paper or a baking mat and then fill the center with jam or jelly. Repeat for all the cookies.
Bake at 325°F for 12-14 minutes. I like these a little underbaked myself, so I tend to stick to around the 12-minute mark.
Notes
For this half and half look, I divided the dough in half (before chilling) and added a few tablespoons of crushed freeze-dried raspberries to the dough. Then I used Nutella for filling instead of raspberry jam. You can customize this with all sorts of other mix-ins or fillings.