Toast hazelnuts in a preheated 350° oven for 6–8 minutes. Allow to cool, then pulse in a food processor with 1/4 cup of brown sugar until finely ground.

In a medium bowl, combine the flour, baking powder, salt and cinnamon.

In a large bowl beat 1/4 cup brown sugar and butter with a hand-mixer until pale and fluffy, about 6 minutes. (About half that amount of time if using a stand-mixer) Add nut mixture and beat until well combined, about 1 minute. Beat in the egg and vanilla. Stir in the flour mixture until partly combined, then beat on low speed to fully combine. Divide dough into 2 and form each half into a flat disk on plastic wrap. Wrap and chill in the fridge for 2 hours.

Prepare a clean counter surface with a large sheet of wax paper, using a small piece of tape on each side to help hold it in place. Sprinkle with flour. Remove one disk of dough from the fridge and place onto prepared surface. very lightly flour the top of the dough and add an additional piece of wax paper on top. Roll out the dough to 1/8" thick, and remove the top layer of wax paper.

Have two cookie sheets ready beside your work area. Using cookie cutter, cut cookies as close together as possible to maximize the dough. If the cookies don't stay in your cutter to transfer onto the cookie sheet and remain on the wax paper instead, gently lift away the excess dough using an offset metal spatula until the edges of the first row of cookies are exposed. Gently slide the offset spatula between the cookie and the wax paper layer, lifting the cookie being careful not to allow it to warp or stretch to maintain the shape. Transfer to cookie sheet, leaving 1" between each cookie. Repeat with remaining dough until your two cookie sheets are full. Transfer the cookie sheets to the fridge to chill for 15 minutes. 

Preheat oven to 350°. Once chilled, remove cookies from fridge and using the smaller heart cutter, cut out the centres from half of the cookies. Gently remove the centres, (I found a toothpick worked well to flick them out while preserving their shape). Drop the small cutouts onto the baking sheet in the spaces between the cookies, distributing on the two sheets if needed. Place in preheated oven and bake for 10 – 12 minutes, until edges are lightly golden. Allow cookies to cool for a few minutes on the cookie sheet, then transfer carefully to a wire rack, keeping the tops separate from the bottoms (and mini-hearts will be too small for the rack, they can cool on a plate).

Asses your dough scraps, press together into a disk and transfer to the fridge to chill. Dough can be re-rolled once or twice max in this manner. Complete cookie cutting and baking process until all cookies are baked and ready.

Once cool, take the wire racks with the top halves with the cutouts and place on a baking sheet. Sift icing sugar on top and set aside.

Spread jam onto the bottom layers, using a generous amount and keeping it in the centre with a 1/8" clear area around the edges of the cookies. As the jam squishes out when you add the tops, this will keep them neat and prevent the jam from oozing out the sides. Carefully add the pretty dusted tops. I like filling the baby heart cookies with jam as well, without any sugar on top.

These cookies are best stored in a tin so they can breathe. Cut 5 or so sheets of wax paper to the size of your tin (I trace the lid as a guide). Line the bottom, add the first row of cookies carefully in a single layer; top with a fitted wax paper sheet, layer on the next row of cookies, and so on, until the tin is full. Sprinkle in the baby hearts as you go (if you don't snack on them all first!)