Using a mixer, beat butter and mascarpone until light and creamy, for about 2 minutes. Add vanilla extract and salt. Add 2-1/2 cups of sugar, half a cup at a time and continue beating until smooth. Add more sugar, if desired.

To assemble, pipe a ring of mascarpone buttercream on top of each brownie (we coated the entire top and it became a bit runny!). Cut off the base of each strawberry so it is flat and place it upside down on each brownie, pushing slightly to secure in place. Top the tip of the strawberry with a dot of buttercream to finish the santa hat. Enjoy!