Marmerkoekjes

No, that is expected at first, since the recipe has you work cold butter cubes into the flour until crumbly. Add the 1-2 teaspoons of cold milk a little at a time and knead just until the dough holds together. Resist adding more liquid than needed, because a wetter dough spreads the marbling and softens the cut-out edges.

You can leave it out and make plain vanilla cookies, but then you lose the marble effect entirely. If you swap the 8 g of cocoa for another dry powder, keep the amount the same so the two dough halves stay the same consistency - otherwise the cocoa half turns dry and cracks when rolled. Dutch-processed and natural cocoa both work here, since the baking powder is in the base dough for both halves.

The most common mistake is over-kneading the two doughs together. Press the vanilla and cocoa halves together and fold them only two or three times - as the recipe says, knead lightly - then stop while you can still see distinct streaks. Rolling out also blends the colors, so roll in as few passes as possible to 5 mm.

Yes. The marbled dough can be wrapped and chilled ahead, though you should let it sit at room temperature briefly before rolling so it doesn't crack. The baked cookies also keep well unfilled - the recipe has you cool them completely before filling anyway, so bake one day and sandwich them with the buttercream the next.

Keep them in an airtight container, since the buttercream is butter-based and picks up fridge odors easily. They are best within a couple of days, because the moisture from the filling gradually softens the crisp cookie. If you want them to stay crisp longer, store the cookies and buttercream separately and assemble shortly before serving.