In the bowl of a stand mixer, combine flour, instant yeast, sugar, and salt. Mix briefly to distribute.

Add the egg, softened butter, and lukewarm milk. Mix on low speed with the dough hook until the dough comes together, about 2 minutes.

Increase to medium speed and knead for 6 to 8 minutes until the dough is smooth, elastic, and slightly tacky but not sticky. It should pull away from the sides of the bowl.

Shape the dough into a rectangle about 8x10 inches. Wrap tightly in plastic wrap and refrigerate for at least 1 hour, or overnight.

Place the cold 225g of butter between two sheets of parchment paper or plastic wrap. Using a rolling pin, pound and roll the butter into a flat rectangle about 5x7 inches.

The butter should be pliable but still cold—bendable without cracking but not soft or greasy. Return to refrigerator until ready to use.

Remove the chilled dough from refrigerator. On a lightly floured surface, roll it into a rectangle approximately 10x14 inches.

Place the butter block in the center of the dough. Fold the top third of dough down over the butter, then fold the bottom third up, like folding a letter. You've now enclosed the butter.

Turn the dough 90 degrees. Roll it out again into a rectangle about 10x14 inches. Fold into thirds again—this is your first turn.

Wrap and refrigerate for 30 minutes.

Remove dough from refrigerator. Roll into a 10x14 inch rectangle again. Fold into thirds (second turn). Wrap and refrigerate 30 minutes.

Repeat one more time: roll, fold into thirds (third turn), wrap, and refrigerate at least 1 hour before using, or overnight.

Whisk together the cornstarch and water in a small bowl to create a slurry. Set aside.

Combine the frozen blueberries, sugar, and lemon juice in a small saucepan over medium heat. Stir occasionally as the berries break down and release their liquid, 3 to 4 minutes.

Once bubbling with berries broken down, stir in the cornstarch slurry. Continue stirring 1 to 2 minutes until the mixture thickens to a jammy consistency.

Remove from heat and cool completely before using. This is essential to prevent soggy bottoms.

Remove the laminated dough from refrigerator. On a lightly floured surface, roll it out to about ⅛ inch thickness.

Using a funnel (open side down) and paring knife, or a 4-inch round cutter, cut circles large enough to line muffin cups with slight overhang. Gather and re-roll scraps as needed, being careful not to overwork.

Gently press each circle into a muffin cup without stretching the dough. The dough should come up the sides with a slight overhang.

Preheat oven to 400°F (200°C).

Spoon 1 to 2 tablespoons cooled blueberry filling into each pastry-lined cup. Don't overfill or filling will bubble over.

Cut remaining pastry scraps into rough pieces to cover tops. Lay pieces over filling and pinch edges together to seal. The rustic, pieced-together look is beautiful!

Brush tops with beaten egg and sprinkle generously with turbinado sugar.

Bake 20 to 25 minutes until puffed and deep golden brown.

Cool in tin 5 minutes before removing to prevent sticking.

Preheat oven to 400°F (200°C) with rack in center position.

Bake 20-25 minutes until puffed and deep golden brown. The tops should be richly colored—don't pull them too early.

Cool in tin 5 minutes, then carefully remove to a wire rack.