In a small bowl, whisk together the flour, baking powder, salt, and cinnamon if using. Set aside.
In a large bowl, beat the butter and brown sugar until light and fluffy. Add the eggs one at a time, mixing well after each. Stir in the vanilla extract.
Add the flour mixture to the butter mixture in batches, alternating with the softened date mixture. Stir gently until just combined.
Grease an 8×8-inch baking dish or similar pan. Pour the batter into the pan and smooth the top. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
While the cake bakes, combine the cream, butter, and brown sugar in a saucepan over medium heat.
Stir until the butter melts and the sugar dissolves. Bring to a gentle simmer and cook for 3–5 minutes, until the sauce thickens slightly.
When the cake comes out of the oven, poke it all over with a toothpick or skewer. Pour about half of the warm toffee sauce over the cake so it soaks in.
Let it sit for 10 minutes before serving.
