1. Make the Dough

• In a stand mixer bowl, whisk together flour, sugar, yeast, salt, lemon zest, and poppy seeds.
• Add warm milk, eggs, and vanilla. Mix with dough hook on low speed until combined.
• Gradually add softened butter in chunks, kneading for 15–20 minutes until dough is smooth and passes the windowpane test.
• Shape into a ball, place in a greased bowl, cover, and let rise in a warm spot for 1–2 hours until doubled.

2. Prepare the Filling (Optional Swirl)

• Mix sugar and lemon zest with fingertips to release oils.
• Blend with softened butter and ground poppy seeds to form a spreadable paste.

3. Shape the Brioche

• Roll dough into a rectangle (~12×18 inches).
• Spread filling evenly (if using), then roll tightly from the long edge.
• Slice lengthwise and braid the two halves, then coil into a round or place in a loaf pan.
• Cover and let rise again for 45 minutes.

4. Bake

• Preheat oven to 350°F (180°C).
• Bake for 35–40 minutes until golden brown and cooked through.
• Cool in pan for 10 minutes, then transfer to wire rack.

5. Glaze

• Mix powdered sugar and lemon juice until smooth.
• Drizzle over cooled brioche and garnish with zest if desired.