1 Beat the butter and sugar: Place the sugar, butter, vanilla, baking powder, and salt in the bowl of stand mixer fitted with a paddle attachment. Beat the ingredients together until the batter is creamy, light in color, and clings to the side of the bowl, about 2 minutes on medium speed.
2 Add the egg: Scrape down the side of the bowl and add the egg. Mix until incorporated.
3 Add the flour: Add the flour and mix it into the dough on low speed until no more streaks of flour remain.
4 Divide and color the dough: Scrape the dough out onto a clean surface and divide in half. Set one half aside. Place the other half back in the bowl, and add the red food coloring and raspberry extract. Mix until incorporated and uniform in color.
Divide each half of the dough in half again – you should have 4 balls of dough, two plain and two raspberry. Roughly shape each ball into a square or rectangle, and tightly wrap each square in plastic wrap.
5 Refrigerate the dough for at least 30 minutes, or up to a day, to let the dough firm.
6 Roll out the dough: Remove one piece of dough, unwrap it, and place on a piece of wax paper or parchment paper. Place another piece of wax paper or parchment paper on top, and roll the dough into a 6 x 12 rectangle. If the dough is too hard to roll out, let it soften for a few minutes until it's easier to roll.
As you roll the dough, occasionally flip it over. If the parchment paper has become wrinkled, unpeel it from the dough and reapply (so you don't get wrinkles in your dough).
When done, transfer the rolled-out dough to a baking sheet, still sandwiched between sheets of paper. Repeat with the remaining 3 pieces of dough, stacking the sheets of dough on top of each other on the baking sheet.
7 Once all four pieces of dough are rolled out, place the cookie sheet with the dough in the freezer for 15 minutes. The dough should be firm, but not frozen through.
8 Sandwich together the vanilla and raspberry doughs: Remove one of the sheets of vanilla dough and one of the sheets of raspberry dough from the freezer. Peel back the top paper on both.
Carefully flip the raspberry dough onto the vanilla dough so they are sandwiched together (vanilla on bottom, raspberry on top – the colors are reversed in the photos below, but they'll give you the general idea!). Peel away the remaining paper from the raspberry dough.
Trim the edges so the two sheets of dough line up. Carefully roll the dough from the long edge to make a 12-inch long log of dough, peeling away the bottom layer of parchment as you go.
9 Repeat with remaining dough: Repeat this sandwiching and rolling process, but this time put the raspberry layer on the bottom so that when you roll the dough, the raspberry dough is on the outside.
10 Chill the logs of dough: Wrap each log of dough tightly in the parchment paper or plastic wrap. Chill in the fridge for 2 hours or overnight. (Or freeze for up to a month.)
11 Heat the oven to 350ºF and line a baking sheet with a piece of parchment paper or silicon baking mat.
12 Slice the cookies: Remove the chilled dough from the fridge. The bottom of the dough will be slightly flat from where it was resting in the fridge. To smooth this out, gently roll the log against the counter with your hands until the edges are smooth again.
Slice the dough into slice 1/4- to 1/2-inch thick. Place them the baking sheet about an inch apart.
13 Bake for 10 to 12 minutes or until the cookies start to brown ever so slightly on the edges.
Transfer baked cookies to a wire rack to cool. Repeat with slicing and baking the remaining dough. Store in an airtight container for up to 3 days.