Adapted from Liv for Cake
Preheat the oven to 175°C and prepare two cake tins by lining them with parchment (or greasing and flouring well if you prefer).
Make the cake: Cream together room temperature butter, sugar and orange zest until pale and fluffy then add vanilla, and eggs one at a time, mixing until incorporated.
Add to the creamed mixture the flour, baking powder, poppy seeds, milk, orange juice, and salt and mix well until all of the ingredients form a batter.
Bake for 25-30 mins until a toothpick inserted into the center comes out mostly clean. Allow to cool completely before frosting.
Make the frosting: Beat butter until smooth then slowly add the powdered sugar, vanilla, and mascarpone cheese until just combined.
Assemble cake as desired.