Prepare the Crumble: In a medium bowl, combine the 60g plain flour, 50g rolled oats, 50g light brown sugar, and a pinch of salt. Add the 60g of cold, cubed butter. Using your fingertips, rub the butter into the dry ingredients until the mixture resembles coarse breadcrumbs. This is known as the rubbing-in method. Set aside.

Preheat and Prep: Preheat your oven to 180°C (160°C fan) and line two large baking trays with parchment paper.

Cream Butter and Sugar: In a large bowl or the bowl of a stand mixer, beat the 150g of softened unsalted butter and 120g of caster sugar together until light, pale, and fluffy. This should take about 3-4 minutes.

Add Wet Ingredients: Add the egg yolk and 1 tsp of vanilla extract to the butter mixture and beat again until fully combined, scraping down the sides of the bowl as needed.

Combine Dry Ingredients: In a separate bowl, whisk together the 220g plain flour, ½ tsp baking powder, and ¼ tsp salt. Gradually add this to the wet ingredients, mixing on a low speed until just combined. Be careful not to overmix.

Shape the Cookies: Roll the dough into small balls, about 2.5cm (1 inch) in diameter, and place them on the prepared baking trays, leaving some space between them for spreading. You should get around 24 cookies.

Create the Well: Using your thumb or the back of a rounded ½ teaspoon, gently press an indentation into the centre of each dough ball. This is where we'll spoon the jam.

Fill and Top: Carefully spoon about ½ teaspoon of raspberry jam into each indentation. Don't overfill, as it will bubble up in the oven. Generously sprinkle the prepared crumble topping over each cookie, covering the jam. What works best for me is to gently press the crumble down so it adheres to the jam and dough.

Bake to Perfection: Bake for 13-15 minutes, or until the edges of the cookies are lightly golden brown and the crumble topping is toasted. The jam should be bubbling gently.

Cool Down: Let the cookies cool on the baking trays for 5 minutes before carefully transferring them to a wire rack to cool completely. They will be soft when they first come out but will firm up as they cool.