• In a medium mixing bowl, combine all purpose flour, bread flour, baking soda, baking powder and salt. Whisk well to thoroughly combine. Set aside.
• In a large mixing bowl, using a handheld electric mixer, beat peanut butter and butter together until smooth.
• Add sugars, and beat to combine. Scrape down sides and bottom of bowl with a rubber spatula.
• Add eggs and vanilla to the bowl, and beat to combine.
• Add flour mixture to wet ingredients. Beat on low speed until most of the flour is incorporated, then beat on medium speed until the mixture is uniform, about 3 - 4 minutes. Using a rubber spatula, fold cookie dough, making sure to scrape the bottom of the bowl, to ensure everything is well combined.
• Use a small cookie scoop to portion roughly 6 dozen balls of cookie dough onto a parchment-lined baking sheet. Roll cookie dough in your hands to form a round ball. Dip the tops of the cookie dough in granulated sugar.
• Return peanut butter cookie balls to baking sheet, cover tightly with plastic wrap and refrigerate for 30 minutes. 
• Preheat oven to 325°F with convection fan // 350° F without fan // 180°C // Gas mark 4.
• Place cookie dough on parchment-lined baking sheets, leaving 2 inches between each one, and bake for 14 minutes. (Bake one sheet at a time for best results. Keep cookie dough in the fridge until ready to bake).
• Remove from oven and let cool on baking sheet for 10 minutes before moving to cooling rack to cool completely. While cookies are cooling, make chocolate buttercream (recipe below).
• When cookies have cooled completely, line them up in pairs, flipping over the bottom of the sandwich cookie.
• Using a piping bag, or a small spatula, fill cookies with about 1 tablespoon of buttercream.
• Place top cookie on buttercream and lightly press to secure in place.
• Store in an airtight container at room temperature for 3 days, or refrigerate for up to 7 days.

Chocolate buttercream

• Combine all ingredients for chocolate buttercream in a medium mixing bowl and beat on medium/medium-high speed for 3 - 4 minutes, until light and fluffy.