Adjust the oven rack to the middle position, pre-heat the oven to 375 ºF (190 ºC) and line a 12-hole muffin tin with muffin cases (paper liners).
In a large bowl, whisk together the gluten free flour blend, almond flour, light brown sugar, baking powder, baking soda, xanthan gum and salt.
Add the butter and, using a hand mixer with the double beaters attachment or a stand mixer with the paddle attachment, work it into the dry ingredients until you get a mixture resembling breadcrumbs.
In a separate bowl, whisk together the milk, yogurt, eggs, vanilla bean paste and lemon juice, and add them to the butter-flour mixture. Whisk them together until you get a thick, smooth batter with no flour clumps.
Mix in the chocolate chips, reserving some for sprinkling on top, until evenly distributed.
Scoop the batter into the muffin cases, dividing it equally between 12 (for standard muffins, cases filled to the brim) or 9 - 10 cases (for jumbo muffins, batter makes a small mound rising above the edge of the muffin cases). Sprinkle with the reserved chocolate chips.
Bake at 375 ºF (190 ºC) for 18 - 20 minutes (standard muffins) or 20 - 22 minutes (jumbo muffins), or until risen, golden brown on top and an inserted toothpick comes out clean or with a few moist crumbs attached.
Leave in the muffin tin for 3 - 4 minutes, then remove from the tin and place onto a wire cooling rack to cool completely.