In a heatproof bowl, combine the sugar and lemon zest. Use your fingertips to rub them together for about 1 minute until the zest is fully incorporated and the sugar is fragrant.

Whisk in the egg yolks and lemon juice until smooth.

Set the bowl over a saucepan of gently simmering water (a double boiler) over medium heat. Whisk constantly for about 10 minutes until the curd thickens enough to coat the back of a spoon and the sugar is fully dissolved. If it's not thickening, whisk in 1 teaspoon of cornstarch or increase the heat slightly.

Remove from heat and immediately whisk in the cubed butter a few cubes at a time. The residual heat will melt them right in.

Strain through a fine mesh strainer into a bowl, cover, and chill in the fridge for at least 1 hour to fully set.

Warm the whole milk to about 110°F (warm to the touch, not hot). Pour into the bowl of a stand mixer and sprinkle the yeast over the top. Let sit for 5 to 10 minutes until bubbly and foamy.

Meanwhile, in a small bowl, combine the ¼ cup of granulated sugar with the 1 tablespoon of lemon zest. Rub together with your fingertips until fragrant and pale yellow.

Add the lemon sugar, egg, and vanilla extract to the activated yeast mixture. Whisk to combine.

Add the bread flour and mix until a shaggy dough forms.

Knead with the dough hook for 5 minutes (or by hand on a lightly floured surface).

Slowly add the room temperature butter, 1 tablespoon at a time, kneading after each addition until fully incorporated.

Knead for another 5 minutes until the dough is smooth, soft, and elastic.

Cover the bowl with plastic wrap or a damp towel and let rise in a warm spot for 1 hour, or until doubled in size.

In a clean mixing bowl, beat the cream cheese, heavy cream, and vanilla extract together for about 2 minutes until smooth and fluffy.

Slowly add the powdered sugar and beat until thick and pipeable.

Transfer to a piping bag and set aside.

In a small bowl, stir together the melted butter, flour, and sugar until it looks like wet sand with crumbly clumps.

Preheat your oven to 350°F and line a baking sheet with parchment paper.

Punch down the risen dough and divide into 6 equal portions (about 110 to 120 grams each if you have a kitchen scale).

Roll each portion into a smooth symmetrical ball and place on the lined baking sheet, leaving room between each one.

Use a 1-cup measuring cup (or your hands or a spoon) to press down on each ball and create a deep indent in the center.

Beat the remaining egg and brush it onto the sides and exposed edges of each bun (not the filling area).

Pipe a generous swirl of cream cheese filling into the indent of each bun.

Top with ½ to 1 tablespoon of chilled lemon curd right over the cream cheese.

Add a small handful of fresh blueberries on top.

Sprinkle the crumble topping generously around the edges of each bun (not on the filling).

Bake at 375°F for 20 to 24 minutes, until the edges are slightly golden brown.

Let cool on the pan for at least 10 minutes (the filling will be VERY hot).

Dust with powdered sugar before serving and enjoy!