Preheat your oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line it with parchment paper, leaving an overhang on the long sides.
In a medium bowl, whisk together the gluten free baking flour, almond flour, baking powder, baking soda, and salt.
In a large bowl, beat the 1/2 cup peanut butter, softened butter, and brown sugar together with an electric mixer until fluffy and well combined, about 3 minutes. Beat in the eggs one at a time, then stir in the vanilla extract and sour cream until smooth.
Fold the dry ingredients into the wet ingredients with a spatula until just combined. The batter will be thick, rich, and smell exactly like peanut butter cookie dough. Fold in the chocolate chips.
Pour the batter into the prepared pan and smooth the top. Bake for 52 to 62 minutes, until the bread is golden and a toothpick inserted in the center comes out with just a few moist crumbs.
Let the bread cool in the pan for 15 minutes, then lift it out using the parchment paper and transfer to a wire rack to cool completely.
Once cooled, whisk together the 1/4 cup peanut butter, powdered sugar, and milk until smooth and pourable. Drizzle it over the top of the bread in a zigzag pattern. Let it set for a few minutes before slicing into thick pieces.