Pre-heat oven to 375 degrees and lightly grease a 6-cavity jumbo muffin tin.
In a medium bowl, whisk together flour, baking powder, and salt.
In the bowl of a stand mixer or in a large bowl with a hand mixer, beat butter, peanut butter, and sugar together until light and fluffy, 3-4 minutes.
Mix in eggs and milk until just combined.
Gently fold dry ingredients into bowl with wet ingredients until just combined (take care not to over-mix).
Fill cavities of muffin tin halfway with batter.
Place two mini Reese's Peanut Butter Cups on top of batter pressing down gently. Continue filling tins with batter until cavities are about ¾ of the way full.
Top each muffin with about 1 Tbsp of streusel topping.
Bake for 16-18 minutes, until toothpick inserted into muffins comes out only slightly sticky.
Cool in muffin tin for a few minutes before moving muffins to a metal rack to cool completely.