Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

Cream together the butter, granulated sugar, and brown sugar in a large bowl until light and fluffy, about 3-5 minutes.

Mix in the egg yolk and vanilla extract until fully combined.

Whisk together the flour and baking powder in another bowl. Gradually add this dry mixture to the wet ingredients, stirring gently until just combined.

Scoop dough using a tablespoon, roll each portion into a ball, and place them on your prepared baking sheets, spacing them about two inches apart.

Press your thumb gently into the center of each ball to create an indentation and fill with blackberry jam.

Bake for 12-15 minutes, or until the edges are golden brown. Cool on wire racks.