• Preheat oven to 370° and grease a muffin tin with butter. Spring a teaspoon of sugar over each muffin tin so that there's some sugar coating each muffin.
• Wash and zest 1 lime. Slice and juice 1-2 limes, depending on how juicy they are. The goal is to have around 1/4 cup lime juice. Set aside both zest and juice .
• Using a handheld mixer on medium-high, cream together butter and sugar until pale and fluffy. About 2-3 minutes.
• Add in eggs and vanilla extract. Mix on medium-low until combined.
• Add plain Greek yogurt, lime zest and juice. Mix until combined.
• Add in flour, salt, baking powder and baking soda and mix until combined.
• Using a spatula or wooden spoon, fold in the quartered blackberries until combined.
• Using a 1/4 cup measuring cup, scoop 1/4 cup of the batter into each muffin tin.

Lime Zest Streusel Topping

• Melt the butter in a bowl and combine it with the flour, sugar, and lime zest. The mixture should be crumbly.
• To the muffins already scooped in the tin, add 1 heaping tablespoon of streusel topping to the top of each muffin.
• Bake the oven for about 20-25 minutes. You'll want to check around the 20 minute mark with a knife or a toothpick (it should come out clean if ready).
• Remove from oven and let cool for 10-15 minutes. Just pop out of the muffin tin and enjoy!