Preheat oven to 350 F. Melt chocolate, oil, and sugar in a sauce pan on med-low heat, stirring well so the ingredients are properly combined. Transfer to mixing bowl and allow to cool slightly. Sift gluten-free flour, cocoa and baking powder into the mixture and stir well. Add the beaten eggs. Stir until well blended.
Stir together butter, confectioners' sugar, peanut butter, salt, and vanilla in another bowl until smooth. Heat peanut butter, if needed, to make it easier to mix in.
Pour half of the brownie batter into prepared pan; spread evenly with a rubber spatula. Drop dollops of peanut butter filling (about 1 1/2 tablespoon each) on top of batter, spacing about 1 inch or so apart. Pour remaining brownie filling on top. Carefully swirl peanut butter filling into batter with a butter knife, running the knife lengthwise and crosswise through layers.
Bake in preheated 350 degree F oven for about 35 minutes. Test with a toothpick in the centre to check that it’s cooked through. Allow to cool completely. To get nice clean cuts, freeze the cooled brownies for 20 minutes before slicing.
To freeze: Layer cooled, cut brownies in a freezer safe contaner separating the layers with waxed paper. Cover, label and freeze.