Preheat the oven to 350°F. Grease a 9x13-inch metal baking pan well.
Beat the softened butter, granulated sugar, and ⅓ cup agave nectar on medium speed for 3 minutes, until pale and fluffy.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, salt, and cinnamon.
Add the dry ingredients to the butter mixture in three additions, alternating with the buttermilk in two additions, mixing on low speed just until combined after each addition.
Pour the batter into the prepared pan and spread evenly. Bake for 32–35 minutes, until golden and a toothpick comes out with a few moist crumbs.
While the cake bakes, melt the butter for the glaze in a small saucepan over medium heat with the brown sugar, agave nectar, and cinnamon. Simmer, whisking, for 2 minutes until slightly thickened. Remove from heat and whisk in the heavy cream.
As soon as the cake comes out of the oven, poke it all over with a wooden skewer, about two-thirds of the way down. Pour half the warm glaze evenly over the top.
Let the cake rest for 15 minutes, poke again in any missed spots, then pour on the remaining glaze.
Let the cake rest at least 20 more minutes before slicing and serving.