Preheat your oven to 325°F (163°C) and line an 8-inch round cake pan with parchment paper.

In a bowl, whisk together flour, cornstarch, and salt until well-combined.

Beat together softened butter and peanut butter until smooth; then add brown sugar and vanilla extract, mixing well.

Gradually combine the dry ingredients into the wet mixture, blending until clumpy.

Fold in optional add-ins like chocolate chips or chopped nuts, if desired.

Transfer dough into the cake pan and press evenly; dock with a fork.

Sprinkle granulated sugar on top and bake for 28-30 minutes until edges are browned.

Cool for 10-15 minutes, then cut into wedges while warm before allowing to cool completely.