Preheat the oven to 320 F / 160 C.
In a non-stick pan, add 4 tablespoons (60 g) of butter, brown sugar and cinnamon and heat over medium-low heat to dissolve the sugar. Set aside.
In a bowl, mix the flour and baking powder.
In the bowl of the robot, beat 9 tablespoons (125 g) of soft butter for 5 minutes with the sugar, lemon zest and vanilla until the mixture whitens.
Add the egg yolks one by one while continuing to beat at medium speed.
Add the mixture of flour and baking powder, then gradually add the milk.
Pour the dough into a 10-inch (25 cm) diameter round cake pan with a high edge, previously buttered and floured.