Preheat the oven to 350°F (177°C). Adjust the oven rack to the center position. Line a 9x13-inch baking pan with parchment paper, leaving some paper hanging over the sides.

In a large bowl, whisk together the flour, baking soda, ginger, cinnamon, allspice, cloves, nutmeg, pepper, and salt. Set this mixture aside.

In a medium bowl, whisk the melted butter, brown sugar, granulated sugar, and molasses until there are no lumps left. Whisk in the egg and vanilla extract.

Pour the butter mixture into the bowl of flour. Use a large spoon or silicone spatula to combine until there are no traces of flour left. The dough will be thick and shiny.

Transfer the dough to the prepared baking pan. Press it down into an even layer.

Bake the bars for 23-26 minutes. The top should be set but still soft. A toothpick inserted in the center should come out mostly clean with a few moist crumbs.

Let the bars cool in the pan on a cooling rack for at least 1 hour.

In a large bowl, use a handheld or stand mixer fitted with a paddle attachment. Beat the cream cheese and softened butter together on medium-high speed for about 2 minutes until smooth. Scrape the bowl as needed.

Add the confectioners’ sugar, ginger, cinnamon, allspice, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed for 2 minutes until creamy.

Spread the frosting over the cooled bars. If you want, add sprinkles on top.

For better results, refrigerate the bars for 30 minutes before cutting.

Lift the bars out of the pan using the parchment paper. Place them on a cutting board and cut into squares.

Store any leftover bars tightly covered in the refrigerator for up to 5 days.