For the blondies:

• Preheat the oven to 325 degrees (F). Line a 9X13-inch baking pan with parchment paper, letting the excess paper hang over the edges of the pan by 1 inch so you can grab the edges and pull the blondies from the pan after they have baked. Spray the parchment with nonstick spray or coat well with butter.
• In a large bowl whisk together the melted butter and sugars until well combined. Add the eggs and vanilla; mix well. Add in the flour and salt and - using a rubber spatula - fold the dry ingredients into the egg mixture until just combined; do not overmix. Pour the batter into the prepared pan, smoothing the top with the spatula. *The batter will be very thick.
• Bake until the top of the bars is light golden brown and slightly firm to the touch; about 25 minutes. Cool on a wire rack to room temperature, then, using a small round cookie cutter, cut the bars into as many rounds as possible. I used a 2" cutter and got exactly 12 blondie bites. Place the blondie bite rounds in the freezer for 10 minutes.

In the meantime, make your frosting (you may choose to make one flavor or both):

For the Chocolate Frosting:

• Sift together the confectioners sugar and cocoa powder, whisking well to ensure it's bump and lump free!
• Using a handheld mixer OR a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until creamy; about 2 minutes. Reduce speed to low and slowly add the sifted sugar/cocoa powder, alternating with the milk, then add the vanilla and salt. Once all of the ingredients have been added, beat on high speed until light and creamy and combined; beat for at least 1 minute on high.

For the Vanilla Buttercream:

• Sift the confectioners sugar, whisking well to ensure it's bump and lump free!
• Using a handheld mixer OR a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until creamy; about 2 minutes. Reduce speed to low and slowly add the sifted sugar, alternating with the milk, then add the vanilla and salt. Once all of the ingredients have been added, beat on high speed until light and creamy and combined; beat for at least 1 minute on high.

Assembly:

• Remove blondie bites from freezer after 10 minutes and transfer them to a large cutting board. Carefully - using a sharp, serrated knife - slice the blondie in half (through the center; not down the middle). Fill the first layer of blondie with 1 tablespoon of frosting, top with the blondie lid, then swirl another dollop of frosting on top. Add a good pinch of sprinkles to the top of each finished blondie bite. Repeat process until all blondies have been stuffed. Serve at once, or keep in the refrigerator/ a cool room until serving. Blondies will keep for 1 day when stored in an airtight container.