My concern was that the appeal of butter cake was more the magic of the moment than the quality of the cookie. After all, it was a spring evening in Amsterdam.

As the sun set, golden beams slid across cobblestones and canals. People emerged—wine and beer in hand—to sit at café tables and on stoeps to enjoy the season’s first breath of warm.

In my hands—a glass of pinot grigio and a small square of butter cake—dense, moist and yellow. A blondie crossed with a sugar cookie.

I’d come to the Netherlands to scour Amsterdam’s best bakeries, a search for a singular Dutch treat worthy of bringing back to Milk Street. The winner wasn’t at first obvious.

That morning, I’d been smitten with a stroopwafel purchased and eaten in the sun outside the gothic, palace-like Centraal Train Station. Thin, crisp waffles sandwiched a thick caramel sauce. It crackled and oozed as I bit.

Then I was drawn by a crowd down a narrow alley to Van Stapele Koekmakerij, where I waited in line for 20 minutes outside the Dickensian shop. Chocolate and sugar wafted around. The shop sells only one thing. A chocolate cookie stuffed with a white chocolate filling. And they are sold only as they bake. Hence the line. Chewy, moist and rich, this was my winner.

Except a few doors down I spotted Banketbakkerij Lanskroon, a 110-year-old bakery at the edge of a canal where I grabbed a vigenwafel, a thinly glazed fig-filled waffle cookie topped with sesame seeds. Sweet and complex, an almost crunchy Fig Newton with a savory side. This!

But then there was Melly’s Cookie Bar, where soaring mint green walls housed a small case of quick breads—chocolate; white chocolate-blueberry; apple-walnut—and alfajores de maicena, a crumbly cornstarch-based sandwich cookie layered with dulce de leche and rimmed with coconut. I don’t even like coconut, but I wanted more.

And there were others. The early morning bakery with mounds of brightly colored meringues; the late night dives where drunken backpackers jostled for marzipan-stuffed gevulde koeken.

Until I sat at the edge of the canal that evening, sun on my face, wine in hand. Taking a bite of that small square of butter cake—or boterkoek, as this classic Dutch treat is known—was transformative. Dense and richly buttery like a cake, yet crumbly, perhaps the offspring of shortbread and a chewy chocolate chip cookie.

It was simple and clean, sweet and rich, but not overwhelming. This was that singular treat.

Back at Milk Street, it was obvious the appeal transcended that moment on the canal. And we loved the simplicity of it—just six ingredients: butter, sugar, flour, an egg, vanilla and a bit of lemon zest. The method is equally simple—butter and sugar are creamed, then the egg and flour are added. Patted into a pan, the top gets a cross-hatch scoring, an easy decorative touch.

As with all things so simple, success was a matter of proportion. And quality of ingredients.

We did add a seventh ingredient—just a bit of salt—which helped balance the sugar and butter. And because the butter is such a prominent player in this recipe, quality mattered. We liked a salted grass-fed Irish butter.