Preheat the oven to 350 degrees Fahrenheit. Line 2 cookie sheets with parchment paper for easier cleanup later, or you can just use an ungreased cookie sheet.
Cream the butter and sugar together in a large bowl until light and fluffy using a standing mixer with a paddle attachment or a hand mixer.
Add the egg and vanilla. Mix to combine.
In a medium bowl, mix the flour, baking powder and salt. Add to the butter mixture and mix just until combined. Do not over mix.
Divide the batter into 4 pieces. Roll each portion of dough into a log, then flatten and shape the log into a 12″ X ¾″ rectangle.
Make an indentation in the center of the rectangle. Fill the center with about 2 Tablespoons of jam. I usually use seedless raspberry jam, but any jam will work in this recipe.
Repeat with the remaining 3 portions of dough. Place 2 sections of shaped and filled rectangles on each cookie sheet.
Bake in the preheated oven for 15-20 minutes, just until the edges are lightly golden. (Most ovens will only need 15 minutes. Do not overbake.)
Cool the cookies for about 5 minutes and then cut them on a diagonal into ¾ inch pieces while still warm.
Store the cookies at room temperature, covered lightly after 1 day, for 3-5 days, but they are best the first few days.
