Preheat the oven to 375 degrees F. Cream the butter and sugar together until fluffy, 3-5 minutes. Scrape the bowl and add the egg and vanilla. Beat again until smooth.
In a separate bowl, mix together the flour, baking powder, and salt. Slowly add the dry mixture into the butter mixture. Scrape the bowl and beating until just mixed through. Measure out 1/2 pound of dough (a little over a 1/4 of the batch) into a separate bowl. Refrigerate the larger amount. Add thick (not runny) fruit jam to the smaller amount of dough. Add a few drops of food coloring. Mix until the color is distributed evenly.
Place the “fruity” mixture in the fridge to chill. (It’s much more important for THIS dough to be cold than the plain dough, so leave it in for at least 30 minutes.)
Meanwhile, roll the plain dough into 1-tablespoon balls. On a floured work surface, press the balls flat with the palm of your hand. Scoop 1 teaspoon of fruity mixture on top of each circle. Pinch up the sides of the circle until the filling is completely enclosed. Flip the balls over, so that the pinched sides are down. Dip the bottom of a drinking glass in sugar and gently press the balls down to 1/2 - 3/4 inch disks. Repeat with the rest.
Place on parchment paper lined cookie sheets, 2 inches apart. Bake for 10 minutes. Then allow the cookies to cool on the baking sheets for 3-5 minutes before moving.
NOTE: For different colors, make multiple batches. I like using raspberry jam and lemon curd the best!