Preheat oven to 170°C (340°F). Line a baking sheet with parchment paper. In a mixing bowl, cream 60 g softened unsalted butter and 50 g granulated sugar until pale and slightly fluffy (about 1–2 minutes with a hand mixer). Add 80 g sifted cake flour, 15 g cornstarch and a pinch (about 1 tsp) fine salt. Use a spatula or your fingertips to rub the mixture together until it forms coarse, pea-to-crumb sized pieces (soboro). Spread the crumbs in an even layer on the lined baking sheet, leaving some gaps so air circulates. Bake for 12–18 minutes, stirring gently once halfway, until the crumbs are dry and lightly golden. Remove and cool completely on a rack; they will crisp as they cool.

Custard cream (pastry cream)

Infuse milk

Pour 500 ml whole milk into a medium saucepan. Split 1 vanilla bean lengthwise and scrape seeds into the milk, then add the pod. Heat milk over medium heat until it just begins to steam and small bubbles form around the edge (do not boil). Remove from heat and let sit 10 minutes to infuse. If using vanilla extract, skip the steep and add it later.

Whisk yolks and dry ingredients

While milk infuses, combine 4 large egg yolks and 80 g granulated sugar in a heatproof bowl. Whisk until the mixture lightens slightly. Whisk in 35 g cornstarch until fully incorporated and smooth.

Temper eggs and thicken

Remove the vanilla pod from the milk. Gradually pour about one-third of the hot milk into the yolk mixture while whisking constantly to temper the eggs. Then pour the tempered yolk mixture back into the saucepan with the remaining milk. Return to medium-low heat and cook, whisking continuously, until the mixture thickens and comes to a gentle boil (about 2–4 minutes). Once thick and glossy, cook for 30–60 seconds more while whisking to cook out any starchy taste.

Finish and cool

Remove from heat. Whisk in 25 g unsalted butter and 1/4 tsp fine salt until butter is melted and fully incorporated, giving a smooth, silky custard. If using vanilla extract, stir in 1 tsp now. Transfer pastry cream to a shallow bowl, press plastic wrap directly on the surface to prevent a skin, and cool to room temperature, then refrigerate at least 2 hours until well-chilled and set. Optionally, for a lighter texture, beat the chilled pastry cream briefly with a spatula or electric whisk before assembly.

Assembly & finishing

Assemble Custard Cream Soboro

If using slices of soft bread or rolls, cut or hollow a small pocket in each or spread a layer on a slice. Spoon or pipe about 60–80 g of chilled custard cream into each bread pocket or onto each slice (adjust to preference). Generously press and sprinkle the cooled soboro crumbs over the exposed custard so the crumbs adhere and form a crunchy, crumbly topping. Repeat for all portions.

Serve and store

Dust lightly with powdered sugar if desired. Serve chilled or at cool room temperature. Store assembled items in an airtight container in the refrigerator and consume within 24–36 hours for best texture (soboro will gradually soften). Unassembled soboro and custard can be stored separately: soboro in an airtight container at room temperature for up to 3 days; pastry cream refrigerated for up to 3 days.