01 - Preheat oven to 350°F and line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - In a medium bowl, whisk together flour, baking powder, salt, cinnamon, ginger, cardamom, allspice, cloves, and black pepper.
03 - In a large bowl, whisk melted butter with brown sugar and granulated sugar until smooth.
04 - Add eggs one at a time, whisking well after each addition, then stir in vanilla extract.
05 - Fold dry ingredients into the wet mixture by hand just until combined; avoid overmixing.
06 - Pour batter into prepared pan and smooth the surface.
07 - Arrange tea biscuits in a single layer atop batter, gently pressing them in; sprinkle with demerara sugar if desired.
08 - Bake for 28 to 32 minutes, until edges are golden and center is just set. A toothpick should come out with a few moist crumbs.
09 - Allow to cool completely in pan before lifting out with parchment and cutting into squares.