Melt the dark chocolate and set it aside to cool to room temperature—this is crucial because adding warm chocolate to butter will cause it to separate and affect your cookie texture. While the chocolate cools, whisk together the flour, baking powder, baking soda, and salt in a small bowl. Having everything measured and ready before you start creaming butter will keep your workflow smooth and prevent overmixing.

In a large bowl, beat the softened butter and sugar together for about 3 minutes until the mixture is light, fluffy, and pale in color. This creaming process incorporates air into the dough, which helps the cookies rise and creates a tender crumb. Don't skip this step—it makes a real difference in your final texture.

Add the cooled melted chocolate, egg, and vanilla to the creamed butter and sugar mixture from Step 2. Stir until everything is well combined and the dough is smooth. The chocolate should be completely cooled before adding to avoid any separation, and I find adding the egg after the chocolate helps create a more cohesive, silky dough.

Gently fold the dry ingredient mixture from Step 1 into the wet mixture from Step 3 in two additions, stirring just until combined after each addition. Pour in the milk while mixing, incorporating it gradually to create a smooth, workable dough. Be careful not to overmix once you add the flour, as this can lead to tough cookies.

Cover the dough and refrigerate for 2-3 hours until it's firm enough to roll out without sticking. This chilling time allows the gluten to relax and the flavors to develop, which results in better-shaped cookies with less spreading during baking.

Preheat your oven to 350°F. On a lightly floured surface, roll the chilled dough to either 1/8 inch thickness for thinner, crispier cookies or 1/4 inch for softer, cake-like cookies. Cut into heart shapes or your desired Valentine's theme shapes and place on a parchment-lined baking sheet. Bake for 10 minutes if rolled thin, or 12 minutes if rolled thicker, until the edges are just set but the centers still have a slight give.

Transfer the cookies to a wire rack to cool completely. Once cooled, sandwich pairs together with strawberry jam and Nutella, or spread either topping on individual cookies. For the best presentation, I like to use a small offset spatula to spread the jam and Nutella evenly, and the cooled cookies hold up beautifully without any crumbling.