Add the flour, brown sugar, and cinnamon in a medium bowl and mix until combined. Pour in the melted butter and mix until crumbs form. Set aside.
In a medium bowl, beat the cream cheese until smooth. Add the egg yolk, vanilla, and sugar and beat until combined. Set aside.
Whisk together flour, baking powder, baking soda, cinnamon, and salt. In another bowl, cream butter with sugar. Beat in eggs one at a time. Gradually stir in buttermilk and vanilla. The mixture may appear curdled, but that is OK. Pour the wet ingredients into the dry ingredients and stir until just combined, being careful not to over-mix. Gently stir in the diced rhubarb.