In a medium bowl add yeast, 1 tsp (5g) sugar, and 1/3 cup (80ml) lukewarm milk. Let it rest for 5 minutes to activate the yeast.
In a large bowl, whisk together the flour, sugar, salt, and lemon zest. Add the remaining milk, dissolved yeast, beaten egg, butter, and vanilla extract. Start kneading for about 5-10 minutes, until smooth and pulls away from the sides of the bowl.
Transfer the dough to an oiled bowl and cover it with plastic wrap.
Let the dough rest for about 1 hour at room temperature (warm space) until doubled in size.
Grease a 10-inch (26cm) springform pan and line it with parchment paper.