Beat butter and cream cheese together until fluffy; beat in lemon zest, vanilla, and salt; add the confectioners’ sugar a little at a time until the frosting reaches a spreadable consistency. Spread evenly over the cake and scatter cranberries over top.
To make the drizzle melt white chocolate in a double broiler or in the microwave; once it is smooth, use a spoon and drizzle over top of the cake.
Cake is easier to cut when slightly chilled; cut into squares, then triangles.