Oatmeal cookies:

Preheat oven to 350° F / 176° C. Line a half-sheet rimmed baking tray with parchment.

In a medium bowl, whisk together flour, rolled oats, baking soda, ground cinnamon, ground ginger, and kosher salt.

In a separate mixing bowl or the bowl of a stand mixer, whip the butter on high speed until fluffy.

Add the sugars, followed by the black treacle, and mix until well combined.

Mix in the egg and vanilla bean paste.

Add the dry ingredients and mix on low speed until just combined.

Cover bowl with clingfilm/plastic wrap and transfer to the refrigerator to chill, at least 30 minutes.

With a 1 1/2-inch cookie scoop, scoop the mixture onto prepared tray, allowing 2 inches of space between the scoops.

Bake in preheated oven for 10-11 minutes, or until puffy and very lightly brown around the edges. Do not overbake.

For perfectly round cookies, quickly reshape with a round metal cutter. (This also helps to give the oatmeal cookies their distinctive cracked tops!)

Allow cookies to cool on the trays for 10 minutes, then transfer to a wire rack to cool completely.

Repeat with remaining cookie dough.

While cookies cool, prepare the filling.

Filling:

In a large bowl or the bowl of a stand mixer, mix the softened butter on high speed until light and fluffy.

Add the confectioner’s sugar, alternating with the milk and mixing well after each addition.

Mix in the vanilla bean paste.

If necessary, add a bit more milk or confectioner’s sugar to adjust the consistency. (The buttercream should be thick rather than runny or watery, but still thin enough to pipe with ease.)

Transfer to a piping bag.

Assembly:

Divide the oatmeal cookies into pairs.

Pipe a generous blob of buttercream on the flat side of half of the cookies.

Top with the remaining cookies, using your hands to gently sandwich the two halves together.

Store leftover cookies in an airtight container at room temperature for up to three days.